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

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

@@ -0,0 +1,31 @@
+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);
+    }
+}