fix: stub cv2 in tests to prevent timeout under pytest-xdist

Add 'cv2' to the list of stubbed modules in conftest.py to prevent
OpenCV import from hanging under pytest-xdist parallel workers.

The perception_cache.py module imports cv2 at the module level, which
can cause deadlocks/hangs when running tests in parallel with pytest-xdist.
By stubbing cv2 in conftest.py, tests get a MagicMock instead of the
real cv2, avoiding the import hang.

Fixes #1336

tests/conftest.py

@@ -33,6 +33,7 @@ for _mod in [
     "sentence_transformers",
     "swarm",
     "swarm.event_log",
+    "cv2",  # OpenCV import can hang under pytest-xdist parallel workers
 ]:
     sys.modules.setdefault(_mod, MagicMock())