test: assert the prefixed model the responses bridge now hands back (#37744)

a369cb0 made the chat-to-responses bridge return the routing prefix on the
model it passes to responses(), so responses() re-resolving the provider is a
no-op instead of stripping a second prefix. It updated the bridge's own unit
tests but not this one, which still asserted the stripped id and has been
failing llm_translation_testing since that change landed.

The provider still receives gpt-5.4: responses() strips the openai/ prefix on
its own resolve, one layer later than this assertion used to sit. The stale
comment claiming the prefix is stripped before routing goes with it.

Author: yuneng-berri

tests/llm_translation/test_openai.py

@@ -1469,7 +1469,6 @@ def test_responses_gpt54_with_xhigh_reasoning():
         mock_responses.assert_called_once()
         request_body = mock_responses.call_args.kwargs
 
-        # The responses prefix should be stripped before routing.
-        assert request_body["model"] == "gpt-5.4"
+        assert request_body["model"] == "openai/gpt-5.4"
         # chat-completions reasoning_effort must map to Responses API reasoning.
         assert request_body["reasoning"] == {"effort": "xhigh"}