chore(build): clear the compiler and xUnit analyzer warnings CI annotates

The CI test job surfaced 10 annotations, but GitHub caps them at 10 per job —
a full rebuild showed 31 nullable warnings plus 26 xUnit analyzer warnings.

Production code:
- CertificateValidator: X509Certificate2(byte[]) is obsolete (SYSLIB0057) ->
  X509CertificateLoader.LoadCertificate.
- AuthenticationMiddleware: IsValidTenantId now carries [NotNullWhen(true)], so
  the guard above BuildTenantAuthority is visible to flow analysis.
- AgentEndpointMigrationRules: the out params are genuinely nullable on the
  false path; netstandard2.0 has no usable [NotNullWhen], so the contract is
  documented instead.
- Constants.ClassifyEspAppsFailure: errorCode is optional by design (the
  "no HRESULT" branch is the timeout fallback) - signature says so now.

Tests: nullable annotations on helper signatures, Assert.Single/DoesNotContain
overloads instead of pre-filtering with Where, and [InlineData(null)] theories
declare nullable parameters. GatherRule helpers now seed Parameters with the
model default (empty) rather than null, matching a deserialized rule.

No behaviour change. 5616 tests pass; solution builds warning-free apart from
the two pre-existing NU1510 pruning notices on the pinned security packages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Configuration/EndpointMigrationTests.cs

@@ -15,7 +15,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Configuration
         private const string CurrentUrl = "https://autopilotmonitor-api-eu.azurewebsites.net";
         private const string TargetUrl = "https://autopilotmonitor-api-us.azurewebsites.net";
 
-        private static AgentConfigResponse Config(string migrateUrl) =>
+        private static AgentConfigResponse Config(string? migrateUrl) =>
             new AgentConfigResponse { MigrateToApiBaseUrl = migrateUrl };
 
         [Fact]

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Configuration/RemoteConfigServiceCacheHardeningTests.cs

@@ -70,6 +70,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Configuration
             // Read the file the production code actually wrote and deserialize with the same lib.
             var raw = File.ReadAllText(_cachePath);
             var persisted = JsonConvert.DeserializeObject<AgentConfigResponse>(raw);
+            Assert.NotNull(persisted);
 
             Assert.False(persisted.UnrestrictedMode);
             Assert.False(persisted.AllowAgentDowngrade);

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Configuration/RemoteConfigServiceRetryTests.cs

@@ -193,7 +193,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Configuration
                     case BackendBehaviour.AlwaysAuthFailure401:
                         throw new BackendAuthException("Unauthorized", 401);
                     case BackendBehaviour.AlwaysReturnNull:
-                        return Task.FromResult<AgentConfigResponse>(null);
+                        return Task.FromResult<AgentConfigResponse>(null!);
                     default:
                         throw new InvalidOperationException("unknown behaviour");
                 }
@@ -204,9 +204,9 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Configuration
 
             // In-memory + per-instance cache so a prior test's successful fetch can't
             // bleed into a later test that expects UsedDefaults.
-            private AgentConfigResponse _cached;
+            private AgentConfigResponse? _cached;
             protected override void CacheConfig(AgentConfigResponse config) { _cached = config; }
-            protected override AgentConfigResponse LoadCachedConfig() => _cached;
+            protected override AgentConfigResponse LoadCachedConfig() => _cached!;
         }
     }
 }

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Integration/ShellCoreToEngineEspExitingTests.cs

@@ -51,7 +51,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Integration
                 logger: logger,
                 helloTracker: null);
 
-            EspExitedEventArgs captured = null;
+            EspExitedEventArgs? captured = null;
             tracker.EspExited += (_, args) => captured = args;
 
             // Source time IS in the past relative to the clock — verify the tracker carries the

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleCollectorGuardTests.cs

@@ -40,7 +40,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
             try { Directory.Delete(_outsideUsers, recursive: true); } catch { /* best effort */ }
         }
 
-        private GatherRuleContext Context(bool unrestricted = false, string imeLogPathOverride = null)
+        private GatherRuleContext Context(bool unrestricted = false, string? imeLogPathOverride = null)
             => new GatherRuleContext(
                 new AgentLogger(_tmp.Path, AgentLogLevel.Info),
                 "sess", "tenant",
@@ -52,13 +52,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
             };
 
         private static GatherRule Rule(string collectorType, string target,
-            Dictionary<string, string> parameters = null) => new GatherRule
+            Dictionary<string, string>? parameters = null) => new GatherRule
             {
                 RuleId = "GATHER-GUARD-TEST",
                 Title = "guard test",
                 CollectorType = collectorType,
                 Target = target,
-                Parameters = parameters,
+                // Match the model default (empty, never null) so the collectors see the
+                // same shape a deserialized rule without parameters gives them.
+                Parameters = parameters ?? new Dictionary<string, string>(),
                 Trigger = "startup",
                 OutputEventType = "guard_test",
             };

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleExecutorEmitModeTests.cs

@@ -192,7 +192,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
                 GatherRuleExecutor.ComputeCanonicalHash(new Dictionary<string, object> { ["v"] = "1" }),
                 GatherRuleExecutor.ComputeCanonicalHash(new Dictionary<string, object> { ["v"] = 1 }));
             Assert.NotEqual(
-                GatherRuleExecutor.ComputeCanonicalHash(new Dictionary<string, object> { ["v"] = null }),
+                GatherRuleExecutor.ComputeCanonicalHash(new Dictionary<string, object> { ["v"] = null! }),
                 GatherRuleExecutor.ComputeCanonicalHash(new Dictionary<string, object> { ["v"] = "<null>" }));
         }
 

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleExecutorPhaseExitTests.cs

@@ -60,7 +60,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
         }
 
         private static GatherRule ExitRule(
-            string id, string triggerPhase, List<string> activePhases = null, string fromPhase = null)
+            string id, string triggerPhase, List<string>? activePhases = null, string? fromPhase = null)
             => new GatherRule
             {
                 RuleId = id,

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleExecutorPhaseScopeTests.cs

@@ -63,7 +63,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
 
         /// <summary>Interval rule with a 1h period — the timer never ticks within a test run,
         /// so these rules exist purely for synchronous scope-matrix assertions.</summary>
-        private static GatherRule ScopeProbeRule(string id, List<string> activePhases = null, string fromPhase = null)
+        private static GatherRule ScopeProbeRule(string id, List<string>? activePhases = null, string? fromPhase = null)
             => new GatherRule
             {
                 RuleId = id,
@@ -78,7 +78,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
                 ActiveFromPhase = fromPhase,
             };
 
-        private static GatherRule StartupRule(string id, List<string> activePhases = null, string fromPhase = null)
+        private static GatherRule StartupRule(string id, List<string>? activePhases = null, string? fromPhase = null)
             => new GatherRule
             {
                 RuleId = id,

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Ime/CmTraceLogParserTests.cs

@@ -27,7 +27,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Ime
         [InlineData("   ")]                                    // whitespace only
         [InlineData("<![LOG[truncated line with no closing")]  // truncated / missing fields
         [InlineData("garbage prefix <![LOG[msg]LOG]!><time=\"06:08:04.8834397\" date=\"2-8-2026\" component=\"C\" context=\"\" type=\"1\" thread=\"1\" file=\"\">")] // leading junk (non-ignorable) fails the StartsWith gate
-        public void TryParseLine_returns_false_without_throwing_for_malformed_input(string line)
+        public void TryParseLine_returns_false_without_throwing_for_malformed_input(string? line)
         {
             var ok = CmTraceLogParser.TryParseLine(line, out var entry);
 

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Ime/ImeLogPatternReDoSTests.cs

@@ -24,12 +24,13 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Ime
     /// </summary>
     public sealed class ImeLogPatternReDoSTests
     {
-        private static ImeLogPattern Pattern(string id, string regex, string action = null) => new ImeLogPattern
+        private static ImeLogPattern Pattern(string id, string regex, string? action = null) => new ImeLogPattern
         {
             PatternId = id,
             Pattern = regex,
             Category = "always",
-            Action = action,
+            // Action is optional on the wire (absent = null); the model declares it non-nullable.
+            Action = action!,
             Enabled = true,
             Parameters = new Dictionary<string, string>(),
         };

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/SystemSignals/AppxFailureAnalyzerTests.cs

@@ -256,7 +256,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
         [InlineData(null, false)]
         [InlineData("", false)]
         [InlineData("0x8007", false)]
-        public void IsAppxFamilyHresult_MatchesOnlyAppxDeploymentRange(string hresult, bool expected)
+        public void IsAppxFamilyHresult_MatchesOnlyAppxDeploymentRange(string? hresult, bool expected)
         {
             Assert.Equal(expected, AppxFailureAnalyzer.IsAppxFamilyHresult(hresult));
         }

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/SystemSignals/ProvisioningFailureSettleWindowTests.cs

@@ -39,7 +39,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
             public List<EspFailureDetectedEventArgs> EspFailures { get; } = new List<EspFailureDetectedEventArgs>();
             public ProvisioningStatusTracker Tracker { get; }
 
-            public Fixture(Func<IReadOnlyList<AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime.AppPackageState>> packageStatesProbe = null)
+            public Fixture(Func<IReadOnlyList<AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime.AppPackageState>>? packageStatesProbe = null)
             {
                 var clock = new VirtualClock(Fixed);
                 var post = new InformationalEventPost(Sink, clock);
@@ -179,7 +179,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
             }");
 
             var captured = f.CapturedEvents();
-            Assert.Single(captured.Where(e => e.EventType == "esp_failure_settle_started"));
+            Assert.Single(captured, e => e.EventType == "esp_failure_settle_started");
         }
 
         // =====================================================================
@@ -214,8 +214,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
             // No terminal fire — the failure was retracted.
             Assert.Empty(f.EspFailures);
 
-            var recovered = Assert.Single(f.CapturedEvents()
-                .Where(e => e.EventType == "esp_failure_settle_recovered"));
+            var recovered = Assert.Single(
+                f.CapturedEvents(), e => e.EventType == "esp_failure_settle_recovered");
             Assert.Equal("AccountSetup", (string)recovered.Data["category"]);
             Assert.Equal("Apps", (string)recovered.Data["failedSubcategory"]);
             Assert.Equal("inProgress", (string)recovered.Data["observedState"]);
@@ -280,7 +280,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
             f.Tracker.TriggerSettleTimerForTest("AccountSetupCategory.Status");
 
             Assert.Single(f.EspFailures);
-            Assert.Empty(f.CapturedEvents().Where(e => e.EventType == "esp_failure_settle_recovered"));
+            Assert.DoesNotContain(f.CapturedEvents(), e => e.EventType == "esp_failure_settle_recovered");
         }
 
         // =====================================================================
@@ -326,8 +326,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
                 ""AppsSubcategory"": {""subcategoryState"":""failed"",""subcategoryStatusText"":""Apps (Error)""}
             }");
 
-            var settleStarted = Assert.Single(f.CapturedEvents()
-                .Where(e => e.EventType == "esp_failure_settle_started"));
+            var settleStarted = Assert.Single(
+                f.CapturedEvents(), e => e.EventType == "esp_failure_settle_started");
             Assert.Equal(1, (int)settleStarted.Data["trackedAppsNotCompletedCount"]);
             var apps = (List<Dictionary<string, object>>)settleStarted.Data["trackedAppsNotCompleted"];
             var app = Assert.Single(apps);
@@ -354,8 +354,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.SystemSignals
                 ""SecurityPoliciesSubcategory"": {""subcategoryState"":""failed"",""subcategoryStatusText"":""Failed""}
             }");
 
-            var settleStarted = Assert.Single(f.CapturedEvents()
-                .Where(e => e.EventType == "esp_failure_settle_started"));
+            var settleStarted = Assert.Single(
+                f.CapturedEvents(), e => e.EventType == "esp_failure_settle_started");
             Assert.False(settleStarted.Data.ContainsKey("trackedAppsNotCompleted"));
             Assert.False(settleStarted.Data.ContainsKey("trackedAppsNotCompletedCount"));
         }

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/DecisionStepProcessorParkedTripwireTests.cs

@@ -44,7 +44,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
         private readonly FakeQuarantineSink _quarantine = new FakeQuarantineSink();
         private readonly FakeSignalIngressSink _sink = new FakeSignalIngressSink();
         private readonly DecisionEngine _engine = new DecisionEngine();
-        private DecisionStepProcessor _sut;
+        private DecisionStepProcessor? _sut;
 
         public DecisionStepProcessorParkedTripwireTests()
         {
@@ -107,7 +107,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
             return (step, signal);
         }
 
-        private FakeSignalIngressSink.PostedSignal FindTripwirePost()
+        private FakeSignalIngressSink.PostedSignal? FindTripwirePost()
         {
             foreach (var posted in _sink.Posted)
             {
@@ -121,6 +121,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
             return null;
         }
 
+        /// <summary>The tripwire post, asserted present — for the assertions that only run
+        /// after <c>WaitForTripwire()</c> has already confirmed one was emitted.</summary>
+        private FakeSignalIngressSink.PostedSignal RequireTripwirePost()
+        {
+            var post = FindTripwirePost();
+            Assert.NotNull(post);
+            return post!;
+        }
+
         private int CountTripwirePosts()
         {
             var count = 0;
@@ -165,9 +174,9 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
             Thread.Sleep(SuppressWait);
             Assert.Equal(1, CountTripwirePosts());
 
-            var post = FindTripwirePost();
+            var post = RequireTripwirePost();
             Assert.Equal(DecisionSignalKind.InformationalEvent, post.Kind);
-            Assert.Equal(nameof(SessionStage.EspAccountSetup), post.Payload["stage"]);
+            Assert.Equal(nameof(SessionStage.EspAccountSetup), post.Payload!["stage"]);
             Assert.Equal("Warning", post.Payload[SignalPayloadKeys.Severity]);
             Assert.Equal("true", post.Payload[SignalPayloadKeys.ImmediateUpload]);
             Assert.Contains("esp_final_exit", post.Payload["signalsSeen"]);
@@ -222,7 +231,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
             sut.ApplyStep(step, signal);
 
             Assert.True(WaitForTripwire());
-            Assert.Equal(DeadlineNames.ClassifierTick, FindTripwirePost().Payload["armedDeadlines"]);
+            Assert.Equal(DeadlineNames.ClassifierTick, RequireTripwirePost().Payload!["armedDeadlines"]);
         }
 
         [Fact]
@@ -237,7 +246,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
             sut.ApplyStep(step, signal);
 
             Assert.True(WaitForTripwire());
-            Assert.Equal(nameof(SessionStage.AwaitingDesktop), FindTripwirePost().Payload["stage"]);
+            Assert.Equal(nameof(SessionStage.AwaitingDesktop), RequireTripwirePost().Payload!["stage"]);
         }
 
         // ===================================================================== Dwell semantics

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/LifecycleEmittersRemoteConfigTests.cs

@@ -256,8 +256,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
                 RemoteConfigFetchOutcome outcome,
                 int configVersion = 0,
                 int attempts = 1,
-                string failureType = null,
-                string failureMessage = null,
+                string? failureType = null,
+                string? failureMessage = null,
                 int? authStatusCode = null)
             {
                 // CurrentConfig is set via the protected SetConfig path — easiest:

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/SignalAdapters/EspFailureDetectedHResultTests.cs

@@ -33,7 +33,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.SignalAdapters
         [InlineData("Network connections (1 of 1 added)", null)]
         [InlineData("", null)]
         [InlineData(null, null)]
-        public void TryExtractErrorCode_extracts_parenthesised_hex_tail(string statusText, string expected)
+        public void TryExtractErrorCode_extracts_parenthesised_hex_tail(string? statusText, string? expected)
         {
             var actual = ProvisioningStatusTracker.TryExtractErrorCode(statusText);
             Assert.Equal(expected, actual);

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/SignalAdapters/ImeLogTrackerAdapterTimingTests.cs

@@ -195,7 +195,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.SignalAdapters
             var all = f.InfoEvents(SharedEventTypes.ScriptCompleted);
             Assert.Equal(2, all.Count);
             Assert.Equal("65.00", all[1].Payload!["durationSeconds"]);
-            Assert.Equal("cycle_including_reporting_latency", all[1].Payload["durationBasis"]);
+            Assert.Equal("cycle_including_reporting_latency", all[1].Payload!["durationBasis"]);
         }
 
         [Fact]

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/BandwidthEstimatorTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Periodic;
 using Xunit;
@@ -19,7 +19,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
         private static List<BandwidthJobSample> Jobs(params BandwidthJobSample[] jobs)
             => new List<BandwidthJobSample>(jobs);
 
-        private static BandwidthJobSample Job(string fileId, long wan, long lan = 0)
+        private static BandwidthJobSample Job(string? fileId, long wan, long lan = 0)
             => new BandwidthJobSample { FileId = fileId, WanBytes = wan, LanBytes = lan };
 
         [Fact]
@@ -43,8 +43,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
             var estimate = estimator.TryBuildEstimate();
             Assert.NotNull(estimate);
             Assert.Equal(1, estimate.WanSampleCount);
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3);
-            Assert.Equal(16.0, estimate.WanMbpsMax.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsMax!.Value, 3);
             Assert.Equal(6_000_000, estimate.WanBytesObserved);
             Assert.Equal("10-50", estimate.Bucket);
         }
@@ -58,7 +58,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
             estimator.AddSnapshot(T0.AddSeconds(3), Jobs(Job("a", 3_000_000), Job("b", 3_000_000)));
 
             var estimate = estimator.TryBuildEstimate();
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3);
         }
 
         [Fact]
@@ -74,7 +74,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
 
             var estimate = estimator.TryBuildEstimate();
             Assert.Equal(2, estimate.WanSampleCount);
-            Assert.Equal(16.0, estimate.WanMbpsMax.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsMax!.Value, 3);
         }
 
         [Fact]
@@ -91,7 +91,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
             estimator.AddSnapshot(T0.AddSeconds(63), Jobs(Job("a", 106_000_000)));
             var estimate = estimator.TryBuildEstimate();
             Assert.Equal(1, estimate.WanSampleCount);
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3);
         }
 
         [Fact]
@@ -130,8 +130,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
             estimator.AddSnapshot(T0.AddSeconds(3), Jobs(Job("a", wan: 6_000_000, lan: 375_000_000)));
 
             var estimate = estimator.TryBuildEstimate();
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3);
-            Assert.Equal(1000.0, estimate.LanMbpsP90.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3);
+            Assert.Equal(1000.0, estimate.LanMbpsP90!.Value, 3);
             Assert.Equal("10-50", estimate.Bucket); // bucket keyed to WAN, not LAN
         }
 
@@ -182,8 +182,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
 
             var estimate = estimator.TryBuildEstimate();
             Assert.Equal(20, estimate.WanSampleCount);
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3); // p90 stays on the steady plateau
-            Assert.Equal(160.0, estimate.WanMbpsMax.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3); // p90 stays on the steady plateau
+            Assert.Equal(160.0, estimate.WanMbpsMax!.Value, 3);
         }
 
         [Fact]
@@ -265,7 +265,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
 
             var estimate = estimator.TryBuildEstimate();
             Assert.Equal(1, estimate.WanSampleCount); // only the finite positive sample survived
-            Assert.Equal(16.0, estimate.WanMbpsP90.Value, 3);
+            Assert.Equal(16.0, estimate.WanMbpsP90!.Value, 3);
             Assert.Equal(0, estimate.WanBytesObserved);
         }
 

src/Backend/AutopilotMonitor.Functions/Middleware/AuthenticationMiddleware.cs

@@ -6,6 +6,7 @@ using Microsoft.Extensions.Logging;
 using Microsoft.IdentityModel.Protocols;
 using Microsoft.IdentityModel.Protocols.OpenIdConnect;
 using Microsoft.IdentityModel.Tokens;
+using System.Diagnostics.CodeAnalysis;
 using System.IdentityModel.Tokens.Jwt;
 using Microsoft.AspNetCore.Http;
 using System.Net;
@@ -248,7 +249,7 @@ public class AuthenticationMiddleware : IFunctionsWorkerMiddleware
     /// missing/garbage tid here prevents an outbound OIDC-metadata fetch to a bogus authority on
     /// malformed-token floods. Loss-free — every legitimate Entra token carries a GUID tid.
     /// </summary>
-    internal static bool IsValidTenantId(string? tid) => Guid.TryParse(tid, out _);
+    internal static bool IsValidTenantId([NotNullWhen(true)] string? tid) => Guid.TryParse(tid, out _);
 
     /// <summary>
     /// Maps a token issuer + tenant id to the OIDC authority to fetch signing keys from. Entra v1.0

src/Backend/AutopilotMonitor.Functions/Security/CertificateValidator.cs

@@ -127,7 +127,7 @@ namespace AutopilotMonitor.Functions.Security
 
                 // Parse certificate from Base64
                 var certBytes = Convert.FromBase64String(certificateBase64);
-                var certificate = new X509Certificate2(certBytes);
+                var certificate = X509CertificateLoader.LoadCertificate(certBytes);
                 var thumbprint = certificate.Thumbprint;
 
                 logger?.LogDebug($"Validating certificate: Subject={certificate.Subject}, Issuer={certificate.Issuer}, Thumbprint={thumbprint}");

src/Shared/AutopilotMonitor.Shared/Constants.cs

@@ -672,9 +672,11 @@ namespace AutopilotMonitor.Shared
             /// </list>
             /// </summary>
             public static (string FailureType, string Message) ClassifyEspAppsFailure(
-                string errorCode, int? espTimeoutMinutes)
+                string? errorCode, int? espTimeoutMinutes)
             {
-                var normalised = string.IsNullOrEmpty(errorCode) ? null : errorCode.ToLowerInvariant();
+                // netstandard2.0's IsNullOrEmpty lacks [NotNullWhen(false)], so the compiler can't
+                // see that the else branch is non-null (CS8602) — assert it.
+                var normalised = string.IsNullOrEmpty(errorCode) ? null : errorCode!.ToLowerInvariant();
 
                 if (string.Equals(normalised, DetectionFailureHResult, System.StringComparison.Ordinal))
                 {

src/Shared/AutopilotMonitor.Shared/Services/AgentEndpointMigrationRules.cs

@@ -27,9 +27,11 @@ namespace AutopilotMonitor.Shared.Services
         /// Validates <paramref name="candidate"/> as a migration target and returns the
         /// normalized base URL (<c>https://{lowercase-host}</c>, no trailing slash) on success.
         /// Rules: absolute https URI, default port, no userinfo/path/query/fragment, host
-        /// matching <see cref="AllowedHostSuffixes"/>.
+        /// matching <see cref="AllowedHostSuffixes"/>. <paramref name="normalized"/> is non-null
+        /// exactly when this returns <c>true</c> (netstandard2.0 has no usable
+        /// <c>[NotNullWhen]</c>, so callers must check the bool themselves).
         /// </summary>
-        public static bool TryNormalizeTarget(string candidate, out string normalized)
+        public static bool TryNormalizeTarget(string candidate, out string? normalized)
         {
             normalized = null;
 
@@ -77,7 +79,8 @@ namespace AutopilotMonitor.Shared.Services
         /// differs from <paramref name="currentBaseUrl"/> (both compared in normalized form —
         /// serving the agent its own current URL is a no-op, not a migration).
         /// </summary>
-        public static bool IsEffectiveMigration(string candidate, string currentBaseUrl, out string normalizedTarget)
+        public static bool IsEffectiveMigration(
+            string candidate, string currentBaseUrl, out string? normalizedTarget)
         {
             normalizedTarget = null;