chore: split .mypy_cache into explicit restore and save steps in workflows

[BobTheBuidler]

Motivation

• Silence the deprecation warning about the combined cache action (the save-always behavior) by following the recommended pattern of separate restore and save steps for the .mypy_cache entry.

Description

• Replace the combined actions/cache@v5 step with actions/cache/restore@v5 for .mypy_cache in .github/workflows/mypy.yaml and .github/workflows/pytest.yaml while preserving the same cache key and path.
• Add an explicit actions/cache/save@v5 step with if: always() after the jobs to ensure the .mypy_cache is saved even on failure in both workflows.
• No changes to cache keys or paths were made, only the cache action split and step names were updated.

Testing

• No automated tests were executed as part of this change because it only updates CI workflow definitions and is validated by CI when the workflows run.

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Codex Task