test: Add integration marker to user/group tests

tests/server/test_users_api.py

@@ -1,5 +1,9 @@
+import pytest
+
 from nextcloud_mcp_server.client import NextcloudClient
 
+pytestmark = pytest.mark.integration
+
 
 async def test_create_and_delete_user(nc_client: NextcloudClient, test_user):
     """Test creating a user and verifying deletion (cleanup by fixture)."""