[Test] Add a Hugging Face cache artifact verifier

[AndreasKaratzas]

Summary

• Add the requested opt-in session fixture for verifying a materialized Hub snapshot or one file beneath it.
• Check Git and LFS content addresses and recorded sizes, and memoize a digest only while the file identity is unchanged.

The fixture is explicit: it does not intercept, repair, or claim to verify every model load.

Validation

• VLLM_TEST_CLEAN_GPU_MEMORY=0 .venv/bin/python -m pytest -q tests/utils_/test_hf_cache.py — 8 passed.
• The same 8 tests passed with the CI-pinned Hugging Face Hub 1.22 implementation.
• Changed-file pre-commit passed.

Buildkite motivation: Multimodal Standard (Qwen2)

Duplicate check: no open PR was found for a Hugging Face cache artifact verifier.

AI assistance: Codex assisted with investigation, implementation, and validation; Andreas directed the design and reviewed the resulting changes.