fix(offline): atomic entry rollback + tidy test assertions

Review follow-up:

- Wrap the `_offline_refcount == 0` setup in a try/except so any failure
  during the cached-constant mutation (including unexpected non-ImportError
  like RuntimeError or AttributeError from a half-initialized module)
  rolls back *all* partial state before re-raising. Without this, a
  mid-setup crash could leave `huggingface_hub.constants.HF_HUB_OFFLINE`
  mutated but the refcount at 0 — a persistent offline flag outliving
  the process.
- Swap ruff-flagged Yoda comparisons in the new test file (SIM300) and
  add a module-level note warning that these tests mutate global state
  and are not safe under cross-process parallelism.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/tests/test_offline_guard.py

@@ -5,6 +5,11 @@ Verifies that the helper mutates the cached module constants in
 ``huggingface_hub.constants`` and ``transformers.utils.hub`` — not just
 ``os.environ`` — and that concurrent users are refcount-coordinated so
 one thread's exit can't strip another thread's offline protection.
+
+NOTE: These tests mutate process-global state in ``huggingface_hub.constants``
+and ``transformers.utils.hub``. They are not safe under cross-process
+parallelism (e.g. ``pytest-xdist`` with ``--dist=loadfile``/``loadscope``);
+run this file serially.
 """
 
 import os
@@ -36,27 +41,27 @@ def test_mutates_cached_huggingface_hub_constant():
     original = _hf_const().HF_HUB_OFFLINE
     with force_offline_if_cached(True, "t"):
         assert _hf_const().HF_HUB_OFFLINE is True
-    assert _hf_const().HF_HUB_OFFLINE == original
+    assert original == _hf_const().HF_HUB_OFFLINE
 
 
 def test_mutates_cached_transformers_constant():
     original = _tf_hub()._is_offline_mode
     with force_offline_if_cached(True, "t"):
         assert _tf_hub()._is_offline_mode is True
-    assert _tf_hub()._is_offline_mode == original
+    assert original == _tf_hub()._is_offline_mode
 
 
 def test_sets_env_variable():
     original = os.environ.get("HF_HUB_OFFLINE")
     with force_offline_if_cached(True, "t"):
-        assert os.environ.get("HF_HUB_OFFLINE") == "1"
-    assert os.environ.get("HF_HUB_OFFLINE") == original
+        assert "1" == os.environ.get("HF_HUB_OFFLINE")
+    assert original == os.environ.get("HF_HUB_OFFLINE")
 
 
 def test_noop_when_not_cached():
     before = _hf_const().HF_HUB_OFFLINE
     with force_offline_if_cached(False, "t"):
-        assert _hf_const().HF_HUB_OFFLINE == before
+        assert before == _hf_const().HF_HUB_OFFLINE
 
 
 def test_nested_contexts_respect_refcount():
@@ -67,7 +72,7 @@ def test_nested_contexts_respect_refcount():
             assert _hf_const().HF_HUB_OFFLINE is True
         # inner exit must not restore while outer is still active
         assert _hf_const().HF_HUB_OFFLINE is True
-    assert _hf_const().HF_HUB_OFFLINE == original
+    assert original == _hf_const().HF_HUB_OFFLINE
 
 
 def test_concurrent_threads_share_offline_window():
@@ -101,8 +106,8 @@ def test_concurrent_threads_share_offline_window():
     t_fast.join(timeout=5)
 
     assert not errors, errors
-    assert observations == [True], "slow thread lost offline protection"
-    assert _hf_const().HF_HUB_OFFLINE == original
+    assert [True] == observations, "slow thread lost offline protection"
+    assert original == _hf_const().HF_HUB_OFFLINE
 
 
 if __name__ == "__main__":