Add unit tests to prevent IPv6 address formatting regression

Tests verify that newClient() correctly formats the gRPC address for:
- IPv4 addresses: 192.168.1.10:50051
- IPv6 addresses: [2a05:d01c:ae4:ee06:1310::3]:50051
- IPv6 loopback: [::1]:50051

This prevents a repeat of the regression where fmt.Sprintf was used
instead of net.JoinHostPort after code refactoring.

Author: rajish

controllers/medusa/common_test.go

@@ -0,0 +1,80 @@
+package medusa
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	cassdcapi "github.com/k8ssandra/cass-operator/apis/cassandra/v1beta1"
+	"github.com/k8ssandra/k8ssandra-operator/pkg/medusa"
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// recordingClientFactory captures the address passed to NewClient for verification.
+type recordingClientFactory struct {
+	lastAddress string
+}
+
+func (f *recordingClientFactory) NewClient(address string) (medusa.Client, error) {
+	f.lastAddress = address
+	return nil, nil
+}
+
+func (f *recordingClientFactory) NewClientWithTLS(address string, secret *corev1.Secret) (medusa.Client, error) {
+	f.lastAddress = address
+	return nil, nil
+}
+
+func TestNewClientAddressFormatting(t *testing.T) {
+	tests := []struct {
+		name            string
+		podIP           string
+		expectedAddress string
+	}{
+		{
+			name:            "IPv4 address",
+			podIP:           "192.0.2.10", // RFC 5737 documentation range
+			expectedAddress: "192.0.2.10:50051",
+		},
+		{
+			name:            "IPv6 address",
+			podIP:           "2001:db8:ae4:ee06:1310::3", // RFC 3849 documentation range
+			expectedAddress: "[2001:db8:ae4:ee06:1310::3]:50051",
+		},
+		{
+			name:            "IPv6 loopback",
+			podIP:           "::1",
+			expectedAddress: "[::1]:50051",
+		},
+	}
+
+	cassdc := &cassdcapi.CassandraDatacenter{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      "dc1",
+			Namespace: "test-ns",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			factory := &recordingClientFactory{}
+			pod := &corev1.Pod{
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      "test-pod",
+					Namespace: "test-ns",
+				},
+				Status: corev1.PodStatus{
+					PodIP: tt.podIP,
+				},
+			}
+
+			_, _ = newClient(context.Background(), nil, cassdc, pod, factory)
+
+			require.NotEmpty(t, factory.lastAddress)
+			assert.Equal(t, tt.expectedAddress, factory.lastAddress)
+		})
+	}
+}