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

tests/Vantage.App.Tests/Fakes/StubProviders.cs

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+using Vantage.Core.Health;
+using Vantage.Core.Metrics;
+using Vantage.Core.Models;
+
+namespace Vantage.App.Tests.Fakes;
+
+public sealed class ThrowingMetricsProvider : IMetricsProvider
+{
+    public const string Message = "host unreachable";
+
+    public Task<HostMetrics> GetMetricsAsync(HostConfig host, CancellationToken cancellationToken) =>
+        throw new InvalidOperationException(Message);
+}
+
+public sealed class FakeHealthChecker : IHealthChecker
+{
+    private readonly HealthCheckResult result;
+
+    public FakeHealthChecker(HealthCheckResult result)
+    {
+        this.result = result;
+    }
+
+    public int Calls { get; private set; }
+
+    public Task<HealthCheckResult> CheckAsync(HealthCheckConfig check, CancellationToken cancellationToken)
+    {
+        Calls++;
+        return Task.FromResult(result);
+    }
+}

tests/Vantage.App.Tests/HeadlessTestApp.cs

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+using Avalonia;
+using Avalonia.Headless;
+using Vantage.App;
+using Vantage.App.Tests;
+
+[assembly: AvaloniaTestApplication(typeof(HeadlessTestApp))]
+
+namespace Vantage.App.Tests;
+
+public static class HeadlessTestApp
+{
+    public static AppBuilder BuildAvaloniaApp() => AppBuilder
+        .Configure<App>()
+        .UseHeadless(new AvaloniaHeadlessPlatformOptions());
+}

tests/Vantage.App.Tests/HeadlessWindowTests.cs

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+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);
+    }
+}

tests/Vantage.App.Tests/Vantage.App.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <LangVersion>latest</LangVersion>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
+    <PackageReference Include="xunit" Version="2.6.6" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6" />
+    <PackageReference Include="Avalonia.Headless.XUnit" Version="11.0.10" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Vantage.App\Vantage.App.csproj" />
+  </ItemGroup>
+
+</Project>