fix: Multiple test fixes and improvements

- Fix mock_gemini_cli.py to use src/aggregate.py (moved to src directory)
- Add wait_for_event method to ApiHookClient for simulation tests
- Fix custom_callback path in app_controller to use absolute path
- Fix test_gui2_parity.py to use correct callback file path

tests/test_gui2_parity.py

@@ -71,7 +71,7 @@ def test_gui2_custom_callback_hook_works(live_gui: Any) -> None:
  assert response == {'status': 'queued'}
  time.sleep(1.5) # Give gui_2.py time to process its task queue
  # Assert that the file WAS created and contains the correct data
- temp_workspace_file = Path('tests/artifacts/live_gui_workspace/tests/artifacts/temp_callback_output.txt')
+ temp_workspace_file = Path('tests/artifacts/temp_callback_output.txt')
  assert temp_workspace_file.exists(), f"Custom callback was NOT executed, or file path is wrong! Expected: {temp_workspace_file}"
  with open(temp_workspace_file, "r") as f:
   content = f.read()