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/Infrastructure/SurrealDbEndpointTests.cs

@@ -0,0 +1,34 @@
+using TarninaHub.Infrastructure.Settings;
+
+namespace TarninaHub.Tests.Infrastructure;
+
+public class SurrealDbEndpointTests
+{
+    [Fact]
+    public void Build_ReturnsRemoteUrl_WhenConfigured()
+    {
+        var settings = new SurrealDbSettings
+        {
+            RemoteUrl = "ws://127.0.0.1:8010/rpc",
+        };
+
+        var endpoint = SurrealDbEndpoint.Build(settings);
+
+        Assert.Equal("ws://127.0.0.1:8010/rpc", endpoint);
+    }
+
+    [Fact]
+    public void Build_ReturnsRocksDbEndpoint_WithForwardSlashes_WhenEmbedded()
+    {
+        var settings = new SurrealDbSettings
+        {
+            DataDirectory = "data",
+        };
+
+        var endpoint = SurrealDbEndpoint.Build(settings);
+
+        Assert.StartsWith("rocksdb://", endpoint);
+        Assert.DoesNotContain('\\', endpoint);
+        Assert.EndsWith("/data", endpoint);
+    }
+}