✅ test(user-management-ui): update user client mock in integration tests

- adjust user client mock structure to match new API client format
- move $get and $post methods under index property to reflect API endpoint structure

packages/user-management-ui/tests/integration/userManagement.integration.test.tsx

@@ -6,8 +6,10 @@ import { UserManagement } from '../../src/components/UserManagement';
 // Mock API client
 vi.mock('../../src/api/userClient', () => ({
   userClient: {
-    $get: vi.fn(),
-    $post: vi.fn(),
+    index: {
+      $get: vi.fn(),
+      $post: vi.fn(),
+    },
     ':id': {
       $put: vi.fn(),
       $delete: vi.fn(),