code: def test_file_not_found_exception() -> None:
      with init_mock_client():
          bucket_name = "mock-bucket"
          backend = GCSArtifactStore(bucket_name)
  
          with pytest.raises(ArtifactNotFound):
              backend.open_reader("not-found-id")
  
imports: from __future__ import annotations
  import contextlib
  import io
  import os
  from typing import Dict
  from typing import Optional
  from typing import TYPE_CHECKING
  from unittest.mock import patch
  import google.cloud.storage
  import pytest
  from optuna.artifacts import GCSArtifactStore
  from optuna.artifacts.exceptions import ArtifactNotFound
label_desc: Use image node_id 4 for calling a global function with example usage: test_file_not_found_exception() without return types