feat: 新增 MinerU 离线 PDF 转换 Markdown 2.3.0，适配 /to_markdown 接口 - #119

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/github-script@v7, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/