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

src/app_controller.py

@@ -573,7 +573,9 @@ class AppController:
 
  def _test_callback_func_write_to_file(self, data: str) -> None:
   """A dummy function that a custom_callback would execute for testing."""
-  with open("test_callback_output.txt", "w") as f:
+  callback_path = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "tests", "artifacts", "temp_callback_output.txt")
+  os.makedirs(os.path.dirname(callback_path), exist_ok=True)
+  with open(callback_path, "w") as f:
    f.write(data)
 
  def _handle_approve_script(self, user_data=None) -> None: