fix(streaming): keep n>1 choices separate in stream_chunk_builder - #36208

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/