Vouch request: fix for #5322 (502 on empty upstream response)

[simin-yuan]

Hi maintainers,
I'm writing to request a vouch so I can open a PR for issue #5322.
I'm an independent researcher working on agent reliability, and I've previously landed two merged PRs in NVIDIA/NeMo-Agent-Toolkit: NVIDIA/NeMo-Agent-Toolkit#2551 and NVIDIA/NeMo-Agent-Toolkit#2553.
The root of this issue is a silent failure case: when the upstream returns zero bytes, the relay sends nothing back to the client, so the client gets no response at all.
My proposed fix only covers the second acceptance criterion: using write_bad_gateway_response to return a 502 response with Connection: close. I'm leaving the first criterion (retrying against a fresh upstream) untouched — that's a design decision better left to the maintainer team.
I'll run fork CI across Windows and Ubuntu to validate the change before opening the PR.
Could you grant a vouch for this work?
Thanks, Simin Yuan