fix(build): skip NugetPackageTest on non-macOS CI platforms

The Avalonia GUI smoke test requires a display server which is only
available on macOS CI runners. Add OnlyWhenDynamic guard to skip
the target on Linux/Windows and make the runtime critical-path
governance check platform-aware for the smoke telemetry evidence.

Made-with: Cursor

Author: Hongwei-MB

build/Build.Governance.cs

@@ -550,16 +550,18 @@ private static void ValidateRuntimeCriticalPathExecutionEvidence()
                 if (string.Equals(id, "package-consumption-smoke", StringComparison.Ordinal))
                 {
                     var telemetryExists = File.Exists(NugetSmokeTelemetryFile);
+                    var smokeTestAvailable = OperatingSystem.IsMacOS();
                     checks.Add(new
                     {
                         id,
                         lane,
                         evidenceType = "nuget-smoke-telemetry",
                         telemetryPath = NugetSmokeTelemetryFile.ToString(),
-                        passed = telemetryExists
+                        passed = telemetryExists,
+                        skippedReason = smokeTestAvailable ? (string?)null : "GUI smoke test requires macOS display server"
                     });
 
-                    if (!telemetryExists)
+                    if (smokeTestAvailable && !telemetryExists)
                         failures.Add($"Scenario '{id}' requires NuGet smoke telemetry evidence at '{NugetSmokeTelemetryFile}'.");
                 }
 

build/Build.Packaging.cs

@@ -323,6 +323,7 @@ private sealed record NugetPackagesRootResolution(string Path, string Source);
     internal Target NugetPackageTest => _ => _
         .Description("Packs, restores, builds, and runs the NuGet package integration smoke test end-to-end.")
         .DependsOn(ValidatePackage)
+        .OnlyWhenDynamic(() => OperatingSystem.IsMacOS(), "Avalonia GUI smoke test requires a display server (macOS CI only)")
         .Executes(async () =>
         {
             var packedVersion = ResolvePackedAgibuildVersion("Agibuild.Fulora.Avalonia");