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
