fix: restore deleted assertion and add performance optimization

- Restored missing 'assert a not in keys' in test_iteration_order_with_deletions
- Added fast-path length check in CopyOnWriteDict.__eq__() for better performance
- Performance optimization is safe: if lengths differ, mappings cannot be equal

Signed-off-by: prakhar-singh1928 <prakhar.singh1928@ibm.com>

Author: prakhar-singh1928

tests/unit/cpex/framework/test_memory.py

@@ -817,6 +817,7 @@ def test_iter_skips_deleted_keys_in_modifications(self):
         keys = list(cow)
         # Should only have b (from original) and c (from modifications, not deleted)
         assert set(keys) == {"b", "c"}
+        assert "a" not in keys
 
     def test_equality_with_empty_dict(self):
         """CopyOnWriteDict with data should not equal empty dict."""