Fix endpoint admission test namespace creation for ROSA

Avoid attempting to create existing namespaces like kube-system
which triggers managed-cluster-validating-webhooks in ROSA.

Check if namespace exists before creation to prevent unnecessary
webhook calls while maintaining same test behavior.

Fixes: OCPBUGS-60654

Author: Amarthya Valija

test/extended/networking/endpoint_admission.go

@@ -186,9 +186,15 @@ func testOneEndpointSlice(oc *exutil.CLI, client kubernetes.Interface, addrType,
 }
 
 func getClientForServiceAccount(adminClient kubernetes.Interface, clientConfig *rest.Config, namespace, name string) (*kubernetes.Clientset, *rest.Config, error) {
-	_, err := adminClient.CoreV1().Namespaces().Create(context.Background(), &corev1.Namespace{ObjectMeta: metav1.ObjectMeta{Name: namespace}}, metav1.CreateOptions{})
-	if err != nil && !errors.IsAlreadyExists(err) {
-		return nil, nil, err
+	_, err := adminClient.CoreV1().Namespaces().Get(context.Background(), namespace, metav1.GetOptions{})
+	if err != nil {
+		if !errors.IsNotFound(err) {
+			return nil, nil, err
+		}
+		_, err = adminClient.CoreV1().Namespaces().Create(context.Background(), &corev1.Namespace{ObjectMeta: metav1.ObjectMeta{Name: namespace}}, metav1.CreateOptions{})
+		if err != nil {
+			return nil, nil, err
+		}
 	}
 
 	_, err = adminClient.CoreV1().ServiceAccounts(namespace).Create(context.Background(), &corev1.ServiceAccount{ObjectMeta: metav1.ObjectMeta{Name: name}}, metav1.CreateOptions{})