test(bff): /health returns 200 Healthy (refs #28)

Add the BFF Api/Tests skeleton (.NET 10, ASP.NET Core minimal API, xUnit
+ WebApplicationFactory) and a failing test asserting GET /health returns
200 with a Healthy body. The endpoint does not exist yet — the test fails
with NotFound, establishing red before the implementation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/bff/Bff.Tests/HealthEndpointTests.cs

@@ -0,0 +1,20 @@
+using System.Net;
+using Microsoft.AspNetCore.Mvc.Testing;
+
+namespace Bff.Tests;
+
+public class HealthEndpointTests(WebApplicationFactory<Program> factory)
+    : IClassFixture<WebApplicationFactory<Program>>
+{
+    [Fact]
+    public async Task Health_endpoint_returns_200_and_reports_healthy()
+    {
+        var client = factory.CreateClient();
+
+        var response = await client.GetAsync("/health");
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadAsStringAsync();
+        Assert.Contains("Healthy", body);
+    }
+}