fix: name the multipart Content-Disposition error codes and return 0 from m.setvar (2.x) - #3675

Process completed with exit code 2.

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, ammaraskar/gcc-problem-matcher@0f9c86f9e693db67dacf53986e1674de5f2e5f28. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/