build: clear all remaining .NET build warnings to zero

- CmTraceLogParser: annotate TryParseLine (string? line, [NotNullWhen(true)]
  out entry), init string properties (CS8618/CS8625/CS8604)
- Shared: internal NotNullWhen polyfill for netstandard2.0; multi-target
  netstandard2.0;net10.0 so net10 consumers use the BCL attribute and the
  polyfill cannot collide via InternalsVisibleTo (CS0433)
- AgentEndpointMigrationRules.TryNormalizeTarget: now carries [NotNullWhen(true)]
- Functions.csproj: drop DataProtection/Cryptography.Xml security pins made
  redundant by .NET 10 framework-package pruning (NU1510)
- Functions.Tests.csproj: add Microsoft.AspNetCore.App FrameworkReference so
  pruning applies there too and the pins stay unnecessary (NU1903/NU1904)
- Agent tests: nullable test-helper param, Assert.NotNull before dereference

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -107,6 +107,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Ime
             var ok = CmTraceLogParser.TryParseLine(line, out var entry);
 
             Assert.True(ok);
+            Assert.NotNull(entry);
             Assert.Equal(
                 new DateTime(2026, 7, 29, 11, 46, 19, DateTimeKind.Utc).AddTicks(226610).AddMinutes(expectedOffsetMinutes),
                 entry.Timestamp);

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

@@ -16,7 +16,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
     /// </summary>
     public sealed class TenantIdWaitResolutionTests
     {
-        private static AgentConfiguration Build(BootstrapConfigFile bootstrapConfig, params string[] args) =>
+        private static AgentConfiguration Build(BootstrapConfigFile? bootstrapConfig, params string[] args) =>
             AutopilotMonitor.Agent.V2.Program.BuildAgentConfiguration(
                 args: args,
                 tenantId: "tenant-t",

src/Backend/AutopilotMonitor.Functions.Tests/AutopilotMonitor.Functions.Tests.csproj

@@ -8,6 +8,11 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <!-- Same compile context as the product project: with the shared framework referenced,
+         NuGet prunes framework-provided transitive packages (e.g. the vulnerable
+         DataProtection/Cryptography.Xml 10.0.0 pulled in via Microsoft.Identity.Web),
+         which would otherwise surface NU1903/NU1904 advisories here. -->
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.2" />

src/Backend/AutopilotMonitor.Functions/AutopilotMonitor.Functions.csproj

@@ -55,12 +55,6 @@
     <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
     <PackageReference Include="Microsoft.Identity.Web" Version="4.3.0" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.10" />
-    <!-- Security pins: Microsoft.Identity.Web 4.3.0 pulls these transitively at 10.0.0, which carries
-         known advisories (GHSA-9mv3-2cwr-p262 DataProtection; GHSA-37gx-xxp4-5rgx and siblings on
-         Cryptography.Xml). Force the patched 10.0.10 servicing build. Remove once Identity.Web ships
-         a dependency floor past the fix. -->
-    <PackageReference Include="Microsoft.AspNetCore.DataProtection" Version="10.0.10" />
-    <PackageReference Include="System.Security.Cryptography.Xml" Version="10.0.10" />
     <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="8.0.25" />
     <PackageReference Include="Resend" Version="0.2.1" />
   </ItemGroup>

src/Shared/AutopilotMonitor.Shared/AutopilotMonitor.Shared.csproj

@@ -1,12 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <!-- netstandard2.0 for the net48 agent; net10.0 so nullable-enabled backend consumers
+         compile against the real BCL nullability attributes instead of the internal
+         netstandard2.0 polyfill (NullableAttributes.cs), which would otherwise collide
+         with System.Runtime's public copy through InternalsVisibleTo (CS0433). -->
+    <TargetFrameworks>netstandard2.0;net10.0</TargetFrameworks>
     <LangVersion>9.0</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
-  <ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
     <PackageReference Include="System.Text.Json" Version="9.0.1" />
   </ItemGroup>
 

src/Shared/AutopilotMonitor.Shared/Logging/CmTraceLogParser.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text.RegularExpressions;
 
@@ -10,10 +11,10 @@ namespace AutopilotMonitor.Shared.Logging
     public class CmTraceLogEntry
     {
         public DateTime Timestamp { get; set; }
-        public string Message { get; set; }
-        public string Component { get; set; }
+        public string Message { get; set; } = string.Empty;
+        public string Component { get; set; } = string.Empty;
         public int Type { get; set; } // 1=Info, 2=Warning, 3=Error
-        public string Thread { get; set; }
+        public string Thread { get; set; } = string.Empty;
     }
 
     /// <summary>
@@ -42,11 +43,12 @@ namespace AutopilotMonitor.Shared.Logging
         /// Attempts to parse a single line of CMTrace-formatted log.
         /// Returns true if parsing succeeded, false if the line doesn't match the format.
         /// </summary>
-        public static bool TryParseLine(string line, out CmTraceLogEntry entry)
+        public static bool TryParseLine(string? line, [NotNullWhen(true)] out CmTraceLogEntry? entry)
         {
             entry = null;
 
-            if (string.IsNullOrEmpty(line) || !line.StartsWith("<![LOG["))
+            // netstandard2.0's IsNullOrEmpty lacks [NotNullWhen(false)] — assert non-null after it.
+            if (string.IsNullOrEmpty(line) || !line!.StartsWith("<![LOG["))
                 return false;
 
             var match = CmTraceRegex.Match(line);

src/Shared/AutopilotMonitor.Shared/NullableAttributes.cs

@@ -0,0 +1,16 @@
+// netstandard2.0 ships none of the nullable-flow attributes, so shared Try-pattern APIs
+// could not tell nullable-enabled consumers (backend net10, agent test projects) that an
+// out param is non-null on success. Internal polyfill — the consuming compiler matches
+// these attributes by full name from metadata, so accessibility does not matter.
+#if NETSTANDARD2_0
+namespace System.Diagnostics.CodeAnalysis
+{
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        public bool ReturnValue { get; }
+    }
+}
+#endif

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

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace AutopilotMonitor.Shared.Services
 {
@@ -28,10 +29,9 @@ namespace AutopilotMonitor.Shared.Services
         /// 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"/>. <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).
+        /// exactly when this returns <c>true</c>.
         /// </summary>
-        public static bool TryNormalizeTarget(string candidate, out string? normalized)
+        public static bool TryNormalizeTarget(string candidate, [NotNullWhen(true)] out string? normalized)
         {
             normalized = null;