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.

tests/EntKube.Clusters.Tests/Domain/KubernetesClusterTests.cs

@@ -0,0 +1,90 @@
+using EntKube.Clusters.Domain;
+using FluentAssertions;
+
+namespace EntKube.Clusters.Tests.Domain;
+
+public class KubernetesClusterTests
+{
+    [Fact]
+    public void Register_WithValidInputs_CreatesClusterInPendingState()
+    {
+        // Arrange & Act — A tenant admin registers a new cluster with its name and API URL.
+
+        KubernetesCluster cluster = KubernetesCluster.Register(
+            "production-eu",
+            "https://k8s.example.com:6443",
+            "secret-ref-123");
+
+        // Assert — The cluster should be created with a unique ID and start in Pending state
+        // because we haven't verified connectivity yet.
+
+        cluster.Id.Should().NotBe(Guid.Empty);
+        cluster.Name.Should().Be("production-eu");
+        cluster.ApiServerUrl.Should().Be("https://k8s.example.com:6443");
+        cluster.KubeConfigSecret.Should().Be("secret-ref-123");
+        cluster.Status.Should().Be(ClusterStatus.Pending);
+        cluster.RegisteredAt.Should().BeCloseTo(DateTimeOffset.UtcNow, TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public void Register_WithEmptyName_ThrowsArgumentException()
+    {
+        // Arrange & Act — Attempting to register a cluster without a name should fail
+        // because every cluster needs a human-readable identifier.
+
+        Action act = () => KubernetesCluster.Register("", "https://k8s.example.com", null);
+
+        // Assert
+
+        act.Should().Throw<ArgumentException>()
+            .WithParameterName("name");
+    }
+
+    [Fact]
+    public void Register_WithEmptyApiServerUrl_ThrowsArgumentException()
+    {
+        // Arrange & Act — Without an API URL, we can't communicate with the cluster.
+
+        Action act = () => KubernetesCluster.Register("my-cluster", "", null);
+
+        // Assert
+
+        act.Should().Throw<ArgumentException>()
+            .WithParameterName("apiServerUrl");
+    }
+
+    [Fact]
+    public void MarkConnected_SetsStatusToConnected()
+    {
+        // Arrange — A freshly registered cluster in Pending state.
+
+        KubernetesCluster cluster = KubernetesCluster.Register("test", "https://api.test", null);
+
+        // Act — The health check confirms the cluster is reachable.
+
+        cluster.MarkConnected();
+
+        // Assert
+
+        cluster.Status.Should().Be(ClusterStatus.Connected);
+        cluster.LastHealthCheckAt.Should().NotBeNull();
+    }
+
+    [Fact]
+    public void MarkUnreachable_SetsStatusToUnreachable()
+    {
+        // Arrange
+
+        KubernetesCluster cluster = KubernetesCluster.Register("test", "https://api.test", null);
+        cluster.MarkConnected();
+
+        // Act — The health check fails.
+
+        cluster.MarkUnreachable();
+
+        // Assert
+
+        cluster.Status.Should().Be(ClusterStatus.Unreachable);
+        cluster.LastHealthCheckAt.Should().NotBeNull();
+    }
+}