ci: fix WindowsOnly test skip with runtime SkipAttribute

TUnit's --treenode-filter doesn't support compound filters with & or AND
operators reliably across platforms. Instead of CI filtering:

1. Add SkipOnNonWindowsAttribute (extends TUnit's SkipAttribute)
   - Runtime check: OperatingSystem.IsWindows()
   - Auto-skips WindowsOnly tests on non-Windows

2. Bitmap test classes now use both attributes:
   - [WindowsOnly] - CategoryAttribute for categorization/documentation
   - [SkipOnNonWindows] - SkipAttribute for runtime skip

3. Simplified CI workflow:
   - Single --treenode-filter for OpenBugs only (all platforms)
   - WindowsOnly handled at runtime by SkipOnNonWindows
   - Clean job names without platform-specific filters

Author: Nucs

.github/workflows/build-and-release.yml

@@ -44,20 +44,14 @@ jobs:
     # NOTE: Test project currently only targets net10.0. See TODO in NumSharp.UnitTest.csproj
     # to re-enable net8.0 when TUnit compatibility is resolved.
     #
-    # Test filtering (TUnit --treenode-filter syntax):
-    #   - OpenBugs: excluded on all platforms (known-failing bug reproductions)
-    #   - WindowsOnly: excluded on non-Windows (require GDI+/System.Drawing.Common)
+    # Test filtering:
+    #   - OpenBugs: excluded via --treenode-filter (known-failing bug reproductions)
+    #   - WindowsOnly: auto-skipped at runtime via [SkipOnNonWindows] attribute
     - name: Test (net10.0)
-      shell: bash
       run: |
-        if [[ "$RUNNER_OS" == "Windows" ]]; then
-          FILTER='/*/*/*/*[Category!=OpenBugs]'
-        else
-          FILTER='/*/*/*/*[(Category!=OpenBugs) AND (Category!=WindowsOnly)]'
-        fi
         dotnet run --project test/NumSharp.UnitTest/NumSharp.UnitTest.csproj \
           --configuration Release --no-build --framework net10.0 \
-          -- --treenode-filter "$FILTER" --report-trx
+          -- --treenode-filter '/*/*/*/*[Category!=OpenBugs]' --report-trx
 
     - name: Upload Test Results
       uses: actions/upload-artifact@v4

test/NumSharp.UnitTest/NumSharp.Bitmap/BitmapExtensionsTests.cs

@@ -8,6 +8,7 @@
 namespace NumSharp.UnitTest
 {
     [WindowsOnly]
+    [SkipOnNonWindows]
     public class BitmapExtensionsTests : TestClass
     {
         // ================================================================

test/NumSharp.UnitTest/NumSharp.Bitmap/BitmapWithAlphaTests.cs

@@ -8,6 +8,7 @@
 namespace NumSharp.UnitTest
 {
     [WindowsOnly]
+    [SkipOnNonWindows]
     public class BitmapWithAlphaTests : TestClass
     {
         [Test]

test/NumSharp.UnitTest/OpenBugs.Bitmap.cs

@@ -56,6 +56,7 @@ namespace NumSharp.UnitTest
     /// </summary>
     [OpenBugs]
     [WindowsOnly]
+    [SkipOnNonWindows]
     public class OpenBugsBitmap : TestClass
     {
 

test/NumSharp.UnitTest/Utilities/SkipOnNonWindowsAttribute.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Threading.Tasks;
+using TUnit.Core;
+
+namespace NumSharp.UnitTest.Utilities;
+
+/// <summary>
+/// Skips the test on non-Windows platforms (Linux, macOS).
+/// Used for tests requiring GDI+/System.Drawing.Common.
+///
+/// This is separate from <see cref="WindowsOnlyAttribute"/> which is a CategoryAttribute
+/// for CI filtering. Use both together:
+/// <code>
+/// [WindowsOnly]           // CategoryAttribute for CI filtering
+/// [SkipOnNonWindows]      // SkipAttribute for runtime skip
+/// public class BitmapTests { }
+/// </code>
+/// </summary>
+public class SkipOnNonWindowsAttribute : SkipAttribute
+{
+    public SkipOnNonWindowsAttribute() : base("Requires Windows (GDI+/System.Drawing.Common)") { }
+
+    public override Task<bool> ShouldSkip(TestRegisteredContext context)
+        => Task.FromResult(!OperatingSystem.IsWindows());
+}