fix(proxy/gateway): buffer non-streaming upstream bodies before replying

A non-streaming response is one JSON document, but the gateway was forwarding it
chunk by chunk. When the upstream failed mid-body — HTTP/2 stream reset, GOAWAY,
connection loss under concurrent subagent load — the client had already received
a 200 header, so the failure surfaced as a truncated gzip payload and the
ZlibError reported in #897 instead of an error the client could retry.

Read the whole body before writing any header. On a read failure, retry the
upstream once; if that also fails, return a clean 502 the client can act on. The
64MB buffer cap only bounds memory — a provider JSON document never approaches
it — and streaming responses are untouched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>