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.Clusters/Domain/KubernetesCluster.cs

@@ -0,0 +1,76 @@
+namespace EntKube.Clusters.Domain;
+
+/// <summary>
+/// A KubernetesCluster represents a registered cluster that EntKube manages.
+/// It holds the connection details, health state, and metadata needed to
+/// interact with the cluster's API server. This is the aggregate root for
+/// all cluster-related operations.
+/// </summary>
+public class KubernetesCluster
+{
+    public Guid Id { get; private set; }
+    public string Name { get; private set; } = string.Empty;
+    public string ApiServerUrl { get; private set; } = string.Empty;
+    public ClusterStatus Status { get; private set; }
+    public string? KubeConfigSecret { get; private set; }
+    public DateTimeOffset RegisteredAt { get; private set; }
+    public DateTimeOffset? LastHealthCheckAt { get; private set; }
+
+    private KubernetesCluster() { }
+
+    /// <summary>
+    /// Registers a new cluster in the platform. At this point, we know the cluster
+    /// exists and we have connection details — but we haven't verified connectivity yet.
+    /// The cluster starts in a Pending state until a health check confirms it's reachable.
+    /// </summary>
+    public static KubernetesCluster Register(string name, string apiServerUrl, string? kubeConfigSecret)
+    {
+        if (string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Cluster name is required.", nameof(name));
+        }
+
+        if (string.IsNullOrWhiteSpace(apiServerUrl))
+        {
+            throw new ArgumentException("API server URL is required.", nameof(apiServerUrl));
+        }
+
+        return new KubernetesCluster
+        {
+            Id = Guid.NewGuid(),
+            Name = name,
+            ApiServerUrl = apiServerUrl,
+            KubeConfigSecret = kubeConfigSecret,
+            Status = ClusterStatus.Pending,
+            RegisteredAt = DateTimeOffset.UtcNow
+        };
+    }
+
+    /// <summary>
+    /// After a successful health check, we mark the cluster as connected.
+    /// This means the platform can now schedule work against this cluster.
+    /// </summary>
+    public void MarkConnected()
+    {
+        Status = ClusterStatus.Connected;
+        LastHealthCheckAt = DateTimeOffset.UtcNow;
+    }
+
+    /// <summary>
+    /// When a health check fails, we mark the cluster as unreachable.
+    /// Existing workloads keep running, but no new provisioning can happen
+    /// until connectivity is restored.
+    /// </summary>
+    public void MarkUnreachable()
+    {
+        Status = ClusterStatus.Unreachable;
+        LastHealthCheckAt = DateTimeOffset.UtcNow;
+    }
+}
+
+public enum ClusterStatus
+{
+    Pending,
+    Connected,
+    Unreachable
+}