OHH-5: tighten proxy HTTP compliance in rttp_client

[fewensa]

Summary

• confirm the direct sync and async TCP paths already use socket2 as the low-level transport
• make blocking request writes complete reliably with write_all
• send Proxy-Authorization for authenticated HTTP proxies and parse CONNECT responses through the full header boundary
• add blocking and async coverage for proxy auth and HTTPS tunneling

Testing

• cargo test --all-features