test(cuda.core): cover AccessedBySet read methods (N7)

Per Leo's review on PR #1775 (test_managed_ops.py:1), add a test for
the read side of AccessedBySet: __iter__, __len__, __eq__, __repr__.
These are part of the public set-like API (alongside __contains__,
add(), discard(), and the setter, which are already covered) but
were untested.

The cu12 batch fallback path (Leo's other coverage point) is now
exercised by TestPrefetchBatch.test_same_location and
test_per_buffer_location running on cu12 CI — the
cuMemPrefetchBatchAsync skip was dropped in d75a7bd when the
fallback landed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cuda_core/tests/memory/test_managed_ops.py

@@ -595,6 +595,33 @@ def test_accessed_by_add_discard(self, init_cuda):
         finally:
             plain.close()
 
+    def test_accessed_by_read_methods(self, init_cuda):
+        """Cover __iter__, __len__, __eq__, __repr__ on AccessedBySet."""
+        device = Device()
+        _skip_if_managed_location_ops_unsupported(device)
+        device.set_current()
+        plain = DummyUnifiedMemoryResource(device).allocate(_MANAGED_TEST_ALLOCATION_SIZE)
+        try:
+            buf = ManagedBuffer.from_handle(plain.handle, plain.size, owner=plain)
+
+            # Empty initially
+            assert len(buf.accessed_by) == 0
+            assert list(buf.accessed_by) == []
+            assert buf.accessed_by == set()
+            assert "AccessedBySet" in repr(buf.accessed_by)
+
+            # After add
+            buf.accessed_by.add(device)
+            assert len(buf.accessed_by) == 1
+            assert list(buf.accessed_by) == [device]
+            assert buf.accessed_by == {device}
+            assert buf.accessed_by != frozenset()
+
+            # __eq__ vs another AccessedBySet on the same buffer
+            assert buf.accessed_by == buf.accessed_by
+        finally:
+            plain.close()
+
     def test_accessed_by_set_assignment(self, init_cuda):
         device = Device()
         _skip_if_managed_location_ops_unsupported(device)