test(tray): add WindowsFactAttribute; skip DPAPI test on non-Windows

SettingsManager_ProtectsElevenLabsApiKeyForStorage calls
ProtectSettingSecret, which intentionally throws PlatformNotSupportedException
on non-Windows (DPAPI is Windows-only). The test had no platform guard, so
it produced a spurious failure on every Linux CI run.

Adds WindowsFactAttribute following the same pattern as IntegrationFactAttribute:
a custom FactAttribute subclass that sets Skip automatically when the test runs
on a non-Windows platform. Applies it to the one DPAPI test.

The test continues to run and assert fully on Windows CI (windows-latest).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/OpenClaw.Tray.Tests/SettingsRoundTripTests.cs

@@ -215,7 +215,7 @@ public class SettingsRoundTripTests
         Assert.Null(SettingsData.FromJson("not json at all"));
     }
 
-    [Fact]
+    [WindowsFact]
     public void SettingsManager_ProtectsElevenLabsApiKeyForStorage()
     {
         var protectedValue = SettingsManager.ProtectSettingSecret("elevenlabs-key");

tests/OpenClaw.Tray.Tests/WindowsFactAttribute.cs

@@ -0,0 +1,19 @@
+using Xunit;
+
+namespace OpenClaw.Tray.Tests;
+
+/// <summary>
+/// Marks a test that can only run on Windows (e.g. tests that exercise
+/// Windows Data Protection API, NTFS reparse points, or other Win32 surfaces).
+/// The test is automatically skipped on non-Windows platforms.
+/// </summary>
+public sealed class WindowsFactAttribute : FactAttribute
+{
+    public WindowsFactAttribute()
+    {
+        if (!OperatingSystem.IsWindows())
+        {
+            Skip = "Windows-only: requires a Windows platform API.";
+        }
+    }
+}