test: add BadRequestError to the fake openai module fixture

_transcribe_openai now imports BadRequestError for the container-retry
path; the managed-gateway fake module needs to provide it.

tests/tools/test_managed_media_gateways.py

@@ -169,6 +169,7 @@ def _install_fake_openai_module(captured, transcription_response=None):
         APIError=Exception,
         APIConnectionError=Exception,
         APITimeoutError=Exception,
+        BadRequestError=type("BadRequestError", (Exception,), {}),
     )
     sys.modules["openai"] = fake_module