[codex] Resolve DNS upstream hostnames

[madeye]

Summary

• resolve UDP DNS upstream values with DNS when they are provided as host:port
• keep https:// DNS-over-HTTPS parsing unchanged
• add a localhost:53 regression test that accepts either loopback address family

Validation

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