fix(json,json-cppagent): cache JsonSerializerOptions singletons — plug DynamicMethod LCG leak (+3.2-3.8 MB/h RSS) - #249

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/