ci: add strict mypy type checking - #207

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, astral-sh/setup-uv@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

No file matched to [**/uv.lock,**/requirements*.txt]. The cache will never get invalidated. Make sure you have checked out the target repository and configured the cache-dependency-glob input correctly.