fix(server): support chunked request bodies

[782042369]

Summary

• decode HTTP/1.1 chunked request bodies before JSON parsing
• keep the existing Content-Length path unchanged
• consume optional chunk extensions and trailers

Problem

Reverse proxies may forward POST bodies with Transfer-Encoding: chunked and no Content-Length. The server currently reads an empty body in that case, so valid JSON requests fail locally with 400 invalid JSON before reaching Gemini.

Validation

• python3 -m compileall -q gemini_web2api.py gemini_web2api
• reproduced the failure with a forced chunked generateContent request (400 before the fix)
• verified the same request returns 200 after the fix
• verified a reverse-proxied Google-compatible generateContent request returns 200 after deployment