Address review feedback: when port fallback occurs, maintain mapping from
original requested port to the relay server for proper cleanup and reuse.
- Add relayByOriginalPort map to track original port -> relay
- Update ensureChromeExtensionRelayServer to check both maps
- Update stopChromeExtensionRelayServer to clean up both mappings
- Stop function now uses the relay's actual bound port for auth cleanup
When the Chrome extension relay server fails to bind due to port
conflict (EADDRINUSE), automatically try alternative ports in the
dynamic range (49152-65535) instead of failing immediately.
This resolves issues where stale processes hold onto port 18792
after gateway restarts or crashes.
Fixes potential issues related to #8926, #13867, #17584
