code: def get_active_user(self) -> Optional[str]:
      return cast(
          Optional[str], self._mts._match_call("get_active_user")
      )
  
imports: import contextlib
  import os
  from typing import Any, Iterator, List, Optional, cast
  import responses
  from responses import registries
  import determined
  from determined.common.api import authentication
label_desc: Use image node_id 2 for calling the MockTokenStoreInstance obj's underlying member method code with example usage: obj.get_active_user() and returns: cast