fix(memory): clean atomic write test hygiene

Made-with: Cursor

tests/agent/test_memory_store.py

@@ -170,18 +170,13 @@ class TestHistoryWithCursor:
         tmp_path_obj = store.history_file.with_suffix(".jsonl.tmp")
 
         # Mock os.replace to raise an exception
-        original_replace = __import__('os').replace
-        
         def failing_replace(*args, **kwargs):
             raise RuntimeError("Simulated failure")
 
         monkeypatch.setattr('os.replace', failing_replace)
 
-        try:
+        with pytest.raises(RuntimeError):
             store._write_entries(entries)
-            assert False, "Should have raised"
-        except RuntimeError:
-            pass
 
         # Temp file should be cleaned up
         assert not tmp_path_obj.exists()