fix: 流式 /v1/responses 零输出防护失效（#56，#54 回归）—— 空响应不再谎报 response.completed - #57

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/