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/api_hook_client.py

@@ -72,6 +72,17 @@ class ApiHookClient:
  def clear_events(self) -> list[dict[str, Any]]:
   return self.get_events()
 
+
+ def wait_for_event(self, event_type: str, timeout: int = 5) -> dict[str, Any] | None:
+  start = time.time()
+  while time.time() - start < timeout:
+   events = self.get_events()
+   for ev in events:
+    if ev.get("type") == event_type or ev.get("event_type") == event_type:
+     return ev
+   time.sleep(0.2)
+  return None
+
  def post_gui(self, payload: dict) -> dict[str, Any]:
   """Pushes an event to the GUI's SyncEventQueue via the /api/gui endpoint."""
   return self._make_request('POST', '/api/gui', data=payload) or {}