remove(ai_client): delete unused stub and proxy files

Deleted:
- src/ai_client_stub.py
- src/ai_client_proxy.py

Fixed test imports to use ai_client instead of ai_client_stub.

tests/test_discussion_takes_gui.py

@@ -17,8 +17,8 @@ def app_instance():
   patch('src.app_controller.AppController._prune_old_logs'),
   patch('src.app_controller.AppController.start_services'),
   patch('src.api_hooks.HookServer'),
-  patch('src.ai_client_stub.set_provider'),
-  patch('src.ai_client_stub.reset_session')
+  patch('src.ai_client.set_provider'),
+  patch('src.ai_client.reset_session')
  ):
   app = App()
   app.project = {