[codex] add exec-server health check

[richardopenai]

Summary

Adds a lightweight GET /health probe on the exec-server websocket listener. The listener now performs a small preflight check before the websocket handshake, returns 200 OK with ok\n for the health route, and otherwise falls through to the existing websocket JSON-RPC path.

The preflight routing uses an explicit private enum so future probe routes such as /status can be added without turning the transport into a chain of boolean checks.

Validation

• just fmt
• cargo test -p codex-exec-server
• just fix -p codex-exec-server

[starr-openai]

lets make the HTTP service listen on a dedicated port, enabled via CLI like --status-endpoint-port. Check out the k8s docs for HTTP based health checks.