test: add StringSlice Append coverage - #504

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-go@4dc6199c7b1a012772edbd06daecab0f50c9053c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/