nginx's default client_max_body_size (1m) and proxy_request_buffering (on)
were never overridden for the chat/LangGraph route, only for the upload
route. A long pasted prompt either exceeded the default size limit (413) or
got spooled to a temp file before reaching Gateway, which can fail with a
raw nginx 500 on a non-root local run if that temp directory isn't
writable -- reproduced independently (real nginx 1.28.3, byte-identical
error page and matching error-log Permission denied line) while confirming
fancyboi999's diagnosis in issue #3952.
Mirrors the upload route's existing client_max_body_size / proxy_request_
buffering fix onto /api/langgraph/ in all three places this nginx config is
maintained (Docker prod, local make dev, and the Kubernetes/Helm ConfigMap),
sized smaller (20M) since this route only ever carries JSON chat text, never
binary file uploads.
