- Extract duplicated TCP proxy fallback logic into generic
`with_tcp_proxy_fallback` helper used by both `post_request` and
`http_request_sync`, eliminating code drift risk
- Allow caller-supplied Content-Type to override the default in
`parse_simple_header` instead of silently dropping it
- Take body by reference in `post_request_http` to avoid eager clone
when no fallback is needed
- Wrap entire `tcp_proxy_request` flow (connect + handshake + send +
receive) in an overall timeout to prevent indefinite stalls
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>