[codex] Resolve upstream proxy hostnames

[madeye]

Summary

• accept documented upstream proxy host:port values by resolving hostnames during parsing
• preserve direct SocketAddr parsing for IP literals
• cover bare HTTP, http://, and socks5:// localhost inputs without assuming IPv4 vs IPv6 resolution order

Tests

• cargo test upstream_proxy_parse
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check