Skip SkiaSharp rendering tests on FreeBSD

[Olof-Lagerkvist]

Summary

• Add a NotOnFreeBsdFactAttribute using the existing platform-aware xUnit fact pattern used by Tests/WindowsOnlyFact.cs and Tests/NotOnWindowsFact.cs.
• Apply it to all four native SkiaSharp tests in Rendering.Tests/SkiaRenderingTests.cs.
• Preserve test execution on Windows, Linux, and macOS while reporting these four cases as skipped on FreeBSD.

Reason

The SkiaSharp 4.151.2 managed assembly requires native SkiaSharp version 151.x, but the FreeBSD test environment loads native library version 116.0. These four tests fail under both .NET 10 and the .NET 11 preview. Native FreeBSD rendering support is not currently a project requirement.

Verification

• Confirmed all four [Fact] declarations in SkiaRenderingTests.cs are replaced.
• FreeBSD expected result from the reported run: 122 passed, 5 skipped, 0 failed (including one pre-existing Windows-only skip).
• Runtime tests have not been executed as part of this PR preparation; run dotnet test -c Release -f net10.0 on FreeBSD to validate discovery and skip reporting.