fix(proxy): isolate server-info per workflow attempt and verify proxy PID liveness

[Alphaxiaoteng]

Summary

1. Isolates Responses proxy server-info across workflow re-run attempts (Fixes Server-info state collides across workflow re-run attempts #144): Replaced raw ${{ github.run_id }} in server-info file keying with ${{ github.run_id }}-${{ github.run_attempt }}. This guarantees that self-hosted / persistent runners do not collide with or reuse stale state from prior failed/cancelled workflow attempts.
2. Validates Responses proxy PID liveness and purges stale server-info (Fixes Stale server-info files can make later action invocations reuse a dead Responses proxy #133): Added check-proxy-status subcommand. Instead of blindly assuming any existing file means the proxy is running, it inspects the recorded pid and probes process liveness via process.kill(pid, 0). If the PID is dead, it automatically removes the stale file and triggers a fresh proxy launch.

---

Changes

• action.yml:
  • Keyed server-info file with CODEX_RUN_ID: ${{ github.run_id }}-${{ github.run_attempt }}.
  • Replaced naive [ -s "$SERVER_INFO_FILE" ] check with node "$ACTION_PATH/dist/main.js" check-proxy-status "$SERVER_INFO_FILE".
• src/main.ts:
  • Added check-proxy-status CLI command verifying process liveness and auto-cleaning dead PID state.
• test/proxyLiveness.test.mjs:
  • Added unit tests for alive proxy verification and dead PID detection / stale file purging.

---

Verification

• npm run build completed cleanly.
• npm test passed: 146/146 tests passing.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[Alphaxiaoteng]

recheck