fix(resources): verify NLTK mirror archives with pinned digests

[git-jxj]

Summary

This PR makes fallback NLTK resource downloads fail closed unless the downloaded archive matches a pinned SHA-256 digest.

• Pin a digest for every enabled punkt_tab, stopwords, and averaged_perceptron_tagger_eng mirror URL.
• Pass the URL-specific digest to the existing download helper before extraction.
• Remove the Gitee punkt_tab fallback because it has no trusted digest.
• Add offline regression coverage for digest configuration, digest forwarding, checksum failure, and cleanup.

Root Cause

When a required NLTK resource was absent, check_nltk_data downloaded a ZIP from a mutable third-party mirror and extracted it directly. Although download_url already supports checksum verification, the call did not provide sha256. A valid but replaced or corrupted archive could therefore be installed into the NLTK data directory.

Changes

• Move the NLTK mirror configuration to a module-level MIRROR_MAP with per-URL SHA-256 values.
• Use download_url(url, zip_path, sha256=...) for every enabled mirror.
• Keep extraction after the verified download only; a checksum failure raises and does not invoke ZipFile.
• Verify the three pinned archives again through the configured proxy before this commit:
  • ModelScope punkt_tab: c2b16c23d738effbdc5789d7aa601397c13ba2819bf922fb904687f3f16657ed
  • Gitee stopwords: 15c94179887425ca1bedc265608cab9f27d650211f709bb929e320990a4b01d1
  • NLTK GitHub averaged_perceptron_tagger_eng: 6025f530624335c67d6547d44757b357b4e79bae030a0383e9887a92c1718f0b

Reproduction

On the unmodified base, monkeypatching download_url to record its keyword arguments during check_nltk_data(averaged_perceptron_tagger_eng) records sha256 as not passed. The downloader then skips the checksum comparison and resource_utils extracts the returned archive.

Validation

Targeted resource tests:

python -m pytest tests/test_resource_utils.py -q
6 passed

Adjacent NLTK consumer tests:

python -m pytest tests/benchmark/test_ifeval_language_checkers.py -q
2 passed

CI smoke test:

python -m pytest tests/cli/test_all.py::TestRun::test_ci_lite -q -s -p no:warnings
1 passed

Lint:

PATH=/root/code/evalscope/.venv/bin:$PATH make lint
Passed

All added tests use mocked NLTK and download behavior; no test accesses the network.

Scope

The change is limited to NLTK mirror integrity and its tests. It intentionally reuses the existing checksum-capable download helper and does not include HTTP resume or other download behavior changes.

[Yunnglin]

LGTM!