Bump actions/checkout v4 → v6 and actions/setup-python v5 → v6

[jhamon]

Summary

• Bumps actions/checkout from v4 to v6 and actions/setup-python from v5 to v6 across all workflow files
• Both upgrades move to Node.js 24, resolving the Node.js 20 deprecation warnings in CI

Test plan

• Verify CI workflows pass with the updated action versions

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Note

Low Risk
Low code risk since this only changes GitHub Actions versions, but it can impact CI behavior if actions/checkout@v6 or actions/setup-python@v6 introduce breaking changes or different defaults.

Overview
Updates CI action dependencies by bumping actions/checkout from v4 to v6 across all workflows.

Updates Python runtime setup by bumping actions/setup-python from v5 to v6 in composite actions and install test workflows, keeping existing Python version matrices unchanged.

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