test(avalonia): headless tests for rendering, change events, overlays and theme

[Shewart]

Description

Avalonia headless tests, and a theme fix they found.

• New tests/ShellUI.Native.Avalonia.Tests (xunit v3, Avalonia.Headless.XUnit 12.1.3, Skia). It compiles the Avalonia demo's component files, so it tests the code the CLI ships.
  • Every component renders in light and dark.
  • Change events: values set from code raise nothing; a click or key raises the event once (Switch, Checkbox, Toggle, Slider, Tabs, Pagination, Accordion, Collapsible, RadioGroup, Select).
  • Overlays: Dialog keeps Tab inside and returns focus on Escape; Dropdown closes on a click outside without clicking through, and on an item or Escape; HoverCard stays open while hovered and closes when the pointer leaves.
  • Theme: token brushes follow SetTheme; ThemeChanged fires.
• Added to ShellUI.Native.slnx; the build-avalonia CI job (ubuntu, windows, macOS) runs it.
• Fix: ShellTheme.EnsureInitialized used a static bool, so a second Application never got the theme palettes and every token lookup failed. It now remembers which application it set up. Template synced.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• CI/CD changes

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added comments where necessary
• My changes generate no new warnings

Testing Done

• Headless suite: 155 tests pass, run twice in a row, and in Release.
• Main suite: 1462 tests pass. Solution builds with 0 warnings; format clean.
• Breaking each of these on purpose makes a test fail: the old static theme flag, HoverCard light dismiss, Dropdown click-through, Select's user-input flag.