test(launcher): Campaign LA — headless MainWindow view tests close #399

#398 was a crash on every modal open/close caused by MainWindow's
constructor calling AvaloniaXamlLoader.Load(this) instead of the
generated InitializeComponent() — only InitializeComponent assigns the
x:Name backing fields, so every named control was null and the first
Dispatcher.UIThread.Post callback in OnViewModelPropertyChanged threw
NullReferenceException, killing the process. It reached the user gate
because no test in tests/AcDream.Launcher.Tests (ViewModel-only) ever
constructed a MainWindow. #399 is the process gap that let that class of
defect through 14,012 green tests.

Adds Avalonia.Headless.XUnit 12.1.1 to the launcher test project. Its
net10.0 dependency group targets xunit v3, so the project migrates
xunit 2.9.3 -> xunit.v3 3.2.2 (drop-in: all 54 pre-existing tests compile
and pass unchanged under dotnet test via xunit.runner.visualstudio 3.1.4,
which already supported v1/v2/v3; two call sites needed
TestContext.Current.CancellationToken per the new xUnit1051 analyzer).
TestAppBuilder.cs wires [assembly: AvaloniaTestApplication] to a headless
AppBuilder.Configure<App>() so the real App.axaml FluentTheme is live in
tests.

MainWindowViewTests.cs adds 12 [AvaloniaFact]/[AvaloniaTheory] tests:
- an explicit non-null + type check of every x:Name field the
  code-behind dereferences (ProfilesTree, ServerNameTextBox,
  AccountNameTextBox, CharacterNameTextBox, EditorSubmitButton,
  FirstRunDatDirectoryTextBox, FirstRunCloseButton, UpdateCloseButton)
- a reflection sweep over every x:Name found in MainWindow.axaml, so a
  future named control without a matching non-null field fails loudly
- one open+close round trip per ProfileEditorKind (all seven, including
  Remove), plus the first-run wizard and the update prompt, each pumping
  Dispatcher.UIThread.RunJobs() so the queued focus callback actually
  executes instead of just being asserted vacuously
- a dedicated test for the _focusBeforeModal-restore branch (not just
  the ProfilesTree.Focus() fallback), anchored on a real focusable
  button since ProfilesTree (TreeView) has Focusable="False" under
  FluentTheme — its own tab stops are TreeViewItem rows, so the
  close-path assertions check "no exception escaped the dispatcher"
  rather than "focus landed on ProfilesTree"

Falsification (required evidence): reverting MainWindow's constructor to
AvaloniaXamlLoader.Load(this) and rerunning gives 12 failed / 0 passed —
10 tests throw NullReferenceException at MainWindow.FocusActiveModal,
propagating cleanly out of Dispatcher.UIThread.RunJobs() (confirming
dispatcher exceptions are not silently swallowed), and the 2 reflection
tests fail on an explicit "x:Name 'ProfilesTree' was null after
construction" message. Restoring InitializeComponent() gives 12 passed /
0 failed. Full launcher suite: 66 passed / 0 failed, reproduced on both
Windows and native Ubuntu (WSL, no display/Xvfb — Avalonia.Headless needs
none). AcDream.Launcher.Core.Tests: 317/317 unaffected.

No CI workflow change needed: .github/workflows/headless-portability.yml's
portable-launcher job already runs dotnet test on the launcher test
project on both windows-latest and ubuntu-latest with no display setup,
which is sufficient for Avalonia.Headless.

Closes #399.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.Launcher.Tests/LauncherWindowViewModelTests.cs

@@ -389,7 +389,9 @@ public sealed class LauncherWindowViewModelTests
         viewModel.FirstRunWizardShell.OpenCommand.Execute(null);
 
         Task install = viewModel.FirstRunWizardShell.StartCommand.ExecuteAsync();
-        await entered.Task.WaitAsync(TimeSpan.FromSeconds(5));
+        await entered.Task.WaitAsync(
+            TimeSpan.FromSeconds(5),
+            TestContext.Current.CancellationToken);
         Assert.True(viewModel.FirstRunWizardShell.CancelCommand.CanExecute(null));
         Assert.False(viewModel.FirstRunWizardShell.CanEditInputs);
         viewModel.FirstRunWizardShell.CancelCommand.Execute(null);