tests/utils/test_offline.py::OfflineTests::test_offline_mode was skipped and relied on TRANSFORMERS_OFFLINE. Update to use HF_HUB_OFFLINE=1 and make the test hermetic by warming the cache in a first subprocess run, then running offline with socket mocking to forbid network access.

Affected area:

File: tests/utils/test_offline.py
Test: OfflineTests::test_offline_mode

Problem:

Test was @unittest.skip with a TODO.
Behavior depended on environment set after import and on external cache state.