Fix Envoy port-check reliability on macOS

Use connect_ex() instead of bind() to test port availability — immune
to SO_REUSEADDR false-positives and TIME_WAIT false-negatives. Also add
force_exit on old uvicorn server to skip graceful drain when keep-alive
connections block the port.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>