[DSV4] Split mixed-chunk attention: route decode tokens to the fp8 paged MLA kernel - #30338

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/labeler@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/