BUG: Allow MD5 fingerprints in FIPS mode

[Kkkakania]

PR Summary

All MD5 usage in yt is for dataset/cache identifiers or answer-test fingerprints, not for security. This change passes usedforsecurity=False at every MD5 constructor so yt can run on Python builds that block MD5 in FIPS mode while preserving the existing digest algorithm and stored answer hashes.

The regression test simulates FIPS behavior for byte, None, and nested dictionary hashing paths and verifies that each digest remains unchanged.

Fixes #5172.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

Verification

• python -m pytest yt/utilities/tests/test_answer_testing.py -q (3 passed)
• python -m pytest yt/utilities/tests -q (48 passed, 1 skipped)
• python -m pytest yt/data_objects/tests/test_data_containers.py -q (7 passed, 2 skipped)
• uvx pre-commit run --files ... on all seven changed files (all applicable hooks passed)

AI assistance

OpenAI Codex assisted with issue investigation, implementation, testing, and PR drafting. No private data or generated assets are included.

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