Pin GitHub Actions to full-length commit SHAs - #197

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@a37ce9120846195fa4ece8f58b268e6043cb2f26, actions/setup-python@3542bca2639a428e1796aaa6a2ffef0c0f575566, astral-sh/setup-uv@38f3f104447c67c051c4a08e39b64a148898af3a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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