Sending a message to an arena model failed with "'JSONResponse' object has no attribute 'body_iterator'" whenever the backing provider answered with an HTTP error, so the real error never reached the user. Non-streaming requests on an arena model, such as title and tag generation, broke the same way with "'JSONResponse' object is not a mapping".
The arena wrapper assumed the sub-model call always returns a stream for a streaming request and a dict for everything else, but the OpenAI-compatible router returns a plain response object as soon as the provider answers 4xx or 5xx. Both arms now hand that response straight back, which is exactly what the non-arena path already does, so the existing error handling turns it into the usual error message in the chat.
Verified against a matrix of streaming and non-streaming requests with the sub-model returning a stream, a dict, a JSONResponse and a PlainTextResponse: both crashes are gone and the two success paths are unchanged, including the selected_model_id prelude on the stream.
Fixes#29658
d75a7aaf54