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/test_download_source_task.py

@@ -167,7 +167,7 @@ async def test_finalize_soft_limited_is_noop_without_running_event(db_sync):
 @pytest.mark.integration
 @pytest.mark.asyncio
 async def test_download_source_catches_soft_limit_and_salvages_event(
-    db_sync, monkeypatch,
+    db_sync, monkeypatch, celery_conf,
 ):
     """End-to-end wiring: when the inner run raises SoftTimeLimitExceeded,
     download_source's handler must flip the in-flight event to error
@@ -178,8 +178,7 @@ async def test_download_source_catches_soft_limit_and_salvages_event(
     import backend.app.tasks.download as dl
     from backend.app.models import DownloadEvent
 
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
     # This test exercises the soft-limit salvage path, not the per-platform
     # serialization. Neutralize the Redis lock so it can't defer/recurse under
     # eager mode (and so it doesn't contend with test_platform_lock).