fix(router): clamp least_busy request counter at zero (salvage of #25393, credit @mango766) - #34444

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, azure/setup-helm@1a275c3b69536ee54be43f2070a358922e12c8d4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/