fix: strip connection-nominated proxy headers - #217

Summary

RFC 9110 §7.6.1 requires intermediaries to consume the Connection field and remove every header it names before forwarding a message. The proxy removed the standard hop-by-hop fields, while extension fields named by Connection remained in forwarded requests and responses.

Collect option names from every Connection field value and apply them during request and response filtering. Parsing handles comma-separated values, repeated fields, case-insensitive header names, and optional whitespace while preserving unrelated end-to-end headers.

Test Plan

• cargo test -p agentic-server-core proxy::tests
• cargo test --workspace
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• uvx pre-commit==4.4.0 run --all-files