test(backend): add xUnit tests for the API skeleton

Add tests/TarninaHub.Tests with health controller and SurrealDB endpoint builder tests, and register the test and layer projects in the solution.

tests/TarninaHub.Tests/Controllers/HealthControllerTests.cs

@@ -0,0 +1,19 @@
+using Microsoft.AspNetCore.Mvc;
+using TarninaHub.Controllers;
+
+namespace TarninaHub.Tests.Controllers;
+
+public class HealthControllerTests
+{
+    [Fact]
+    public void Get_ReturnsOkWithHealthyStatus()
+    {
+        var controller = new HealthController();
+
+        var result = controller.Get();
+
+        var ok = Assert.IsType<OkObjectResult>(result.Result);
+        var payload = Assert.IsType<HealthResponse>(ok.Value);
+        Assert.Equal("ok", payload.Status);
+    }
+}