Add unit tests for KubernetesCluster, Tenant, ServiceInstance, and RegisterClusterHandler

- Implement tests for KubernetesCluster including registration, connectivity status, and error handling.
- Create tests for Tenant creation, member management, and status changes.
- Add tests for ServiceInstance provisioning and state management.
- Introduce RegisterClusterHandler tests to validate registration requests and error scenarios.
- Set up project files for new test projects with necessary dependencies.

src/EntKube.SharedKernel/Contracts/ApiResponse.cs

@@ -0,0 +1,26 @@
+namespace EntKube.SharedKernel.Contracts;
+
+/// <summary>
+/// Standard API response envelope used by all EntKube microservices.
+/// Every HTTP response wraps its payload in this structure so clients
+/// always know where to find the data, error messages, and metadata.
+/// </summary>
+public record ApiResponse<T>
+{
+    public bool Success { get; init; }
+    public T? Data { get; init; }
+    public string? Error { get; init; }
+    public DateTimeOffset Timestamp { get; init; } = DateTimeOffset.UtcNow;
+
+    public static ApiResponse<T> Ok(T data) => new()
+    {
+        Success = true,
+        Data = data
+    };
+
+    public static ApiResponse<T> Fail(string error) => new()
+    {
+        Success = false,
+        Error = error
+    };
+}