fix: resolve net48-only nullable warnings (CS8601, CS8602, CS8604)

- CS8601: Add null-forgiving operator on guarded dictionary assignments
  in cmdlets where IsNullOrEmpty check precedes the assignment
- CS8602: Add Assert.NotNull after JObject["data"] in xUnit model tests
  (JToken indexer returns nullable on net48)
- CS8604: Add null-forgiving on guarded arguments in NodeService,
  WaitPveTaskCmdlet, GetPveTemplateCmdlet, and auth test parameters

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/PSProxmoxVE.Core/Services/NodeService.cs

@@ -52,7 +52,7 @@ namespace PSProxmoxVE.Core.Services
             var versionStr = data?["version"]?.ToString();
             if (string.IsNullOrEmpty(versionStr))
                 throw new InvalidOperationException("Failed to retrieve PVE version from API response.");
-            return PveVersion.Parse(versionStr);
+            return PveVersion.Parse(versionStr!);
         }
     }
 }