fix(proxy): preserve partial SOCKS response

[JSap0914]

Summary

• Preserve buffered SOCKS response bytes when a reply arrives across multiple reads.
• Advance the buffer only after a complete message has been parsed.
• Related issue: hyper-util SOCKS5: "failed parsing server response" when the CONNECT reply arrives in multiple TCP segments (partial parse consumes the buffer) hyper#4127

Verification

• Focused proxy test: cargo test --test proxy test_socks_v5_without_auth_works
• Proxy test suite: cargo test --test proxy