test(bg): fix drain-loop backoff test import so it actually collects

The regression test used a bare `from _wakeup_helpers import install_fake_registry`,
which raises ModuleNotFoundError under the repo test runner (rootdir = repo root,
tests is a package). pytest treats that as a COLLECTION error for the file, not a
failure — so the whole suite stayed green while this file's tests silently never
ran, leaving the backoff fix with zero effective coverage.

Use the package-qualified `from tests._wakeup_helpers import install_fake_registry`,
matching the four sibling bg_task_complete suites. Confirmed the import now
resolves from the repo root (the bare form fails).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_bg_task_drain_loop_backoff.py

@@ -18,7 +18,7 @@ from __future__ import annotations
 import queue
 import threading
 
-from _wakeup_helpers import install_fake_registry
+from tests._wakeup_helpers import install_fake_registry
 
 
 class _CountingStop: