Treat incomplete cache groups as misses

[Kevin-Li-2025]

New contributor declaration

• I am not making a trivial change, such as fixing a typo in a comment.

• I have written a PR description following these rules.

• I have run pre-commit run --from-ref origin/main --to-ref HEAD.

• Select one of the following.

  • I have added tests.
    • /test for lit tests
    • /unittest for C++ tests
    • /python/test for end-to-end tests
  • This PR does not need a test because FILL THIS IN.

• Select one of the following.

  • I have not added any lit tests.
  • The lit tests I have added follow these best practices,
    including the "tests should be minimal" section. (Usually running Python code
    and using the instructions it generates is not minimal.)

Summary

Make local and remote cache group reads reject incomplete groups instead of returning a partial mapping. A partial group can make compilation treat a corrupted cache entry as a cache hit, only to fail later when loading the missing binary or IR artifact.

What changed

• Return None from FileCacheManager.get_group() if any recorded child path is missing.
• Return None from RemoteCacheManager.get_group() if the remote backend does not return all recorded children.
• Add regression coverage for both local and remote cache managers.

Testing

• pre-commit run --from-ref origin/main --to-ref HEAD
• Added Python unit coverage in python/test/unit/runtime/test_cache.py

[Kevin-Li-2025]

Thanks for the review and approval. CI is green now and the branch is clean. Could a maintainer please merge this when convenient?