fix(gateway): 补修 #473/#474 合入后仍在的问题 - #486

Follow-up to the pre-merge review notes on #473 and #474, which landed on main (and shipped in v0.7.28 / v0.7.29) without these patches. This PR does not revert either feature.

#474 gzip / error shape

• Decompressing past the inflated size limit now returns a distinct ErrInflatedBodyTooLarge (mapped to 413 request body too large) instead of io.LimitReader's silent EOF. A JSON decoder therefore sees "body too large", not a syntax error from a truncated object, and cannot continue with a cut-off body.
• POST /v1/messages body-decoding failures (unsupported encoding, corrupt gzip, over-limit inflate) use the Anthropic error envelope this endpoint already returns for other request errors, not the OpenAI shape from writeRequestBodyError.

#473 heartbeat / headers / metrics / changelog

• Account and provider are known as soon as the executor picks a route, which is before the wait for the upstream response head. A StreamRouteHint on the request context lets setStreamHeaders include X-Qoder-Account, X-CLI2API-Account, and X-CLI2API-Provider before the heartbeat's WriteHeader. Holding the heartbeat until ChatStreamProxy returns would defeat its purpose.
• After Adopt(), a heartbeat write failure (clientGone()) or a canceled request context is recorded as canceled. The handler no longer writes streaming first and does not treat a disconnect during the heartbeat wait as a normal stream open.
• The v0.7.28 changelog (and the heartbeat comments) no longer say this feature avoids a slow upload. Heartbeat starts only after the request body has been read; it covers waiting for the upstream response head / prefill. An unreleased fragment describes the follow-up behavior.

Tests

• Inflated over-limit: hard error, including incomplete JSON at the cap; exact-limit bodies still succeed.
• /v1/messages unknown encoding and corrupt gzip keep the Anthropic error shape.
• Live stream with an early heartbeat commit still exposes account/provider headers.
• Client disconnect during the heartbeat wait is logged as canceled, never as streaming.

Does not merge, release, or deploy.