Add headless coverage for the ItemsSource sample

[wieslawsoltes]

Summary

• add an isolated Avalonia Headless test project for DockXamlSample
• instantiate the real compiled ItemsSourceExample XAML and mount it in a headless window
• verify the two initial documents render as tabs
• execute AddDocumentCommand and verify the source collection, generated dockables, tab items, and rendered Document 1 header all update

Context

The original April reproduction no longer fails on current master; later compiled-binding/sample changes restored the behavior, but the issue remained open and existing tests exercised only the model pipeline. This PR adds end-to-end regression coverage for the exact sample and user flow.

Validation

• dotnet test tests/DockXamlSample.HeadlessTests/DockXamlSample.HeadlessTests.csproj --no-restore --logger "console;verbosity=minimal" (1 passed)
• dotnet build tests/DockXamlSample.HeadlessTests/DockXamlSample.HeadlessTests.csproj -c Release --no-restore --nologo (0 errors)
• dotnet sln Dock.slnx list includes the new test project
• git diff --check

Fixes #1094