Test fixture: give fake backend mock halo table pointers

step() reads _dst/_src before the launch; without them the failed-launch
test raised AttributeError instead of the intended RuntimeError.

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

tests/validation/multigrid/test_block_stepper.py

@@ -115,6 +115,7 @@ class CpuContracts(unittest.TestCase):
         backend._status = "ready"
         backend._stream = Mock()
         backend._copy = backend._step = Mock()
+        backend._dst, backend._src = Mock(), Mock()
         backend._block, backend._grid, backend._copy_grid = (32, 1, 1), (1, 1, 1), (1, 1)
         backend._allocations = [Mock(), Mock()]
         backend._current, backend._spare = backend._allocations