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/Features/RegisterCluster/RegisterClusterHandler.cs

@@ -0,0 +1,53 @@
+using EntKube.Clusters.Domain;
+using EntKube.SharedKernel.Domain;
+
+namespace EntKube.Clusters.Features.RegisterCluster;
+
+/// <summary>
+/// Handles the registration of a new Kubernetes cluster into the platform.
+/// A tenant admin provides the cluster name, API server URL, and optionally
+/// a kubeconfig secret reference. We validate the input, create the cluster
+/// aggregate, and persist it. The cluster starts in Pending state until the
+/// background health-check service confirms connectivity.
+/// </summary>
+public class RegisterClusterHandler
+{
+    private readonly IClusterRepository repository;
+
+    public RegisterClusterHandler(IClusterRepository repository)
+    {
+        this.repository = repository;
+    }
+
+    public async Task<Result<Guid>> HandleAsync(RegisterClusterRequest request, CancellationToken ct = default)
+    {
+        // Validate that the caller provided the minimum required information.
+        // Without a name and API URL, we cannot register a cluster.
+
+        if (string.IsNullOrWhiteSpace(request.Name))
+        {
+            return Result.Failure<Guid>("Cluster name is required.");
+        }
+
+        if (string.IsNullOrWhiteSpace(request.ApiServerUrl))
+        {
+            return Result.Failure<Guid>("API server URL is required.");
+        }
+
+        // Create the cluster aggregate using the domain factory method.
+        // This encapsulates all the business rules for what a valid new cluster looks like.
+
+        KubernetesCluster cluster = KubernetesCluster.Register(
+            request.Name,
+            request.ApiServerUrl,
+            request.KubeConfigSecret);
+
+        // Persist the new cluster so it can be picked up by the health-check background service.
+
+        await repository.AddAsync(cluster, ct);
+
+        return Result.Success(cluster.Id);
+    }
+}
+
+public record RegisterClusterRequest(string Name, string ApiServerUrl, string? KubeConfigSecret);