A backend error arrives as HTTP 200 with a null body

vLLM returned 422 (it could not fetch an image URL -- Wikimedia 403s server-side fetches). Through the proxy that became 200 with a body of literal null.

A caller gets no status, no error object, nothing to branch on. It crashed the benchmark harness, which is the mild version; a client library would likely return None and fail later somewhere unrelated.

Expected: the upstream status and error body pass through.

Start at the proxy's request and upstream-error handling entry point, then reproduce the vLLM response with a 422 status and error body. Done means the proxy returns that upstream status and body instead of HTTP 200 with a literal null; add or run the relevant request test if one exists.

Written by the indexing model from the issue text.