fix(save_without_update_fields): suppress violations in test files

Test files (test_*.py, *_test.py, /tests/ paths) use .save() for
fixture setup, not concurrent Django model updates — no race condition
risk applies. 521/1241 (42%) of corpus violations were in test files,
including 101/111 flagged in celery/django-celery-beat.

Also extracts _is_test_file() helper for potential reuse.

Adds test: test_save_in_test_file_not_flagged (171 tests total).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Your Name

failure_mode.py

@@ -636,6 +636,19 @@ def _is_constructor_call(node: _ast.expr) -> bool:
     return frozenset(new_obj_save_lines)
 
 
+def _is_test_file(path: str) -> bool:
+    """Return True if path looks like a test file (test_*.py, *_test.py, tests/ dir)."""
+    import os as _os
+    basename = _os.path.basename(path)
+    return (
+        basename.startswith("test_")
+        or basename.endswith("_test.py")
+        or "/test/" in path
+        or "/tests/" in path
+        or "/testing/" in path
+    )
+
+
 def _check_save_without_update_fields(
     call: CallSite,
     models: dict[str, ModelManifest],
@@ -648,6 +661,9 @@ def _check_save_without_update_fields(
     # Non-Django files cannot have Django model .save() calls.
     if not _file_imports_django(call.file):
         return []
+    # Test files: .save() calls are fixture setup, not concurrent-update races.
+    if _is_test_file(call.file):
+        return []
     # Positional args mean this is PIL/file/custom .save(path, format, ...) not Django.
     if call.positional_count >= 1 or call.has_var_args:
         return []

test_checker.py

@@ -608,6 +608,33 @@ def update_name(user_id, name):
     assert v, "Fetched object .save() (UPDATE) must still be flagged"
 
 
+def test_save_in_test_file_not_flagged(tmp_path):
+    # Corpus: celery/django-celery-beat, healthchecks — 521/1241 (42%) of
+    # save_without_update_fields violations are in test_*.py files.
+    # Test fixture setup is not subject to concurrent-update races.
+    _write_src(
+        tmp_path,
+        "test_models.py",  # filename starts with test_ → test file
+        """
+        from django.db import models
+
+        class ScheduleTests:
+            def create_model_interval(self, schedule):
+                interval = IntervalSchedule.from_schedule(schedule)
+                interval.save()  # INSERT fixture — must NOT flag
+                return interval
+
+            def test_update(self):
+                obj = MyModel.objects.get(pk=1)
+                obj.enabled = True
+                obj.save()  # test UPDATE — must NOT flag
+        """,
+    )
+    violations = check_codebase(tmp_path)
+    v = [v for v in violations if v.context == "save_without_update_fields"]
+    assert not v, f"save() in test files must not be flagged: {v}"
+
+
 def test_objects_get_not_flagged_as_optional(tmp_path):
     # Corpus: EvalAI — token = JwtToken.objects.get(user=user); token.refresh_token
     # Model.objects.get() raises DoesNotExist, never returns None.