test: set Celery config through a celery_conf fixture, not monkeypatch

pytest 9.2.0 (released 2026-10-27, changelog #10644) changed how
monkeypatch.setattr restores an attribute the object does not hold in
its own __dict__: undo now deletes it instead of assigning the old value
back. Celery's conf serves every key through __getattr__, so undo called
delattr, the delete raised AttributeError, and pytest skipped the rest
of that test's undos. That made 53 teardown errors across six files
(cleanup API, task-run signals, tasks admin, backup, library audit, the
soft-limit salvage test), and could leak patches into later tests.

CI installs pytest unpinned, so the first run after the release picked it
up: the weekly base refresh of main, 2026-11-01, and every run since. The
gate held, so nothing was published.

tests/conftest.py gains `celery_conf(**values)`: it records each key's
value through conf[key], writes with conf.update(), and restores the
saved values after the test. The six files use it.

Task #6864.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -85,6 +85,29 @@ def db_sync(sync_engine):
             session.rollback()
 
 
+@pytest.fixture
+def celery_conf():
+    """Set Celery config keys for one test: `celery_conf(task_always_eager=True)`.
+    The values they had before the test come back after it.
+
+    Use this, not `monkeypatch.setattr(celery.conf, ...)`. Celery serves config
+    keys through `__getattr__`, so monkeypatch sees no attribute to restore and
+    deletes the key on undo instead; the delete raises, and every later undo of
+    that test is skipped, leaking its patches into the tests after it.
+    """
+    from backend.app.celery_app import celery
+
+    saved: dict[str, object] = {}
+
+    def _set(**values: object) -> None:
+        for key in values:
+            saved.setdefault(key, celery.conf[key])
+        celery.conf.update(values)
+
+    yield _set
+    celery.conf.update(saved)
+
+
 @pytest_asyncio.fixture
 async def app():
     return create_app()