refactor(storage_format): split base Node class into two distinctclasses for leaf and internal nodes and updated the usage in serializer.py and tests. - #340

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: marocchino/sticky-pull-request-comment@773744901bac0e8cbb5a0dc842800d45e9b2b405. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/