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/mock_gemini_cli.py

@@ -135,7 +135,7 @@ def main() -> None:
     "role": "assistant",
     "content": "I will read the first 10 lines of the file."
    }), flush=True)
-   file_path = "aggregate.py"
+   file_path = "src/aggregate.py"
    print(json.dumps({
     "type": "tool_use",
     "name": "read_file",