fix: Fix flaky avalonia test

[kevinferrare]

Description of Changes

Fix flaky avalonia tests

[Copilot]

Pull request overview

Replaces [AvaloniaFact] with the standard xUnit [Fact] attribute on UI-related tests that do not actually need a running Avalonia UI thread, removing the associated window show/close and ProcessUiEvents plumbing in StructureViewUiTests. This eliminates flakiness caused by the headless Avalonia dispatcher.

Changes:

• Switch MouseCaptureTests from [AvaloniaFact] to [Fact] (the backends under test are pure managed/native wrappers, no UI required).
• Switch StructureViewUiTests from [AvaloniaFact] to [Fact] and drop the StructureView window instantiation, ShowWindowAndWait, view.Close(), and ProcessUiEvents() calls that the view-model-only assertions don't need.
• Add explicit using Xunit; import in StructureViewUiTests now that the attribute is [Fact].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/Spice86.Tests/UI/MouseCaptureTests.cs	Drops the unused Avalonia.Headless.XUnit import and replaces every [AvaloniaFact] with [Fact].
tests/Spice86.Tests/UI/StructureViewUiTests.cs	Adds the Xunit using, replaces [AvaloniaFact] with [Fact], and removes the now-unneeded view show/close and UI-pump calls in the view-model-only test paths.