test(app): add app test scaffolding and headless test harness

tests/Vantage.App.Tests/HeadlessWindowTests.cs

@@ -0,0 +1,37 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.VisualTree;
+using Microsoft.Extensions.DependencyInjection;
+using Vantage.App;
+using Vantage.App.ViewModels;
+using Vantage.App.Views;
+using Xunit;
+
+namespace Vantage.App.Tests;
+
+public class HeadlessWindowTests
+{
+    [AvaloniaFact]
+    public async Task MainWindow_LoadsXamlAndBindsViewModel()
+    {
+        var services = new ServiceCollection();
+        services.AddVantageApp(useMockProviders: true);
+        var provider = services.BuildServiceProvider();
+
+        var viewModel = provider.GetRequiredService<MainWindowViewModel>();
+        await viewModel.InitializeAsync();
+
+        var window = new MainWindow { DataContext = viewModel };
+        window.Show();
+
+        Assert.Same(viewModel, window.DataContext);
+        Assert.NotEmpty(viewModel.Hosts);
+
+        var tabControl = window.GetVisualDescendants().OfType<TabControl>().FirstOrDefault();
+        Assert.NotNull(tabControl);
+
+        var listBox = window.GetVisualDescendants().OfType<ListBox>().FirstOrDefault();
+        Assert.NotNull(listBox);
+    }
+}