fix: address PR review feedback

1. get_all(): use LIMIT in Cypher query instead of Python slicing
2. _delete_entities/_add_entities: single commit after loop with
   rollback on failure to prevent partial commits
3. _merge_node(): combine agent_id/run_id into single SET query
   (was up to 3 round-trips, now always 1)
4. Add close() method for connection cleanup
5. Extract _get_similar_nodes() helper to deduplicate similarity logic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/memory/test_apache_age_memory.py

@@ -181,13 +181,18 @@ class TestGetAll:
         assert results[0]["relationship"] == "KNOWS"
         assert results[0]["target"] == "bob"
 
-    def test_respects_limit(self):
+    def test_passes_limit_to_cypher(self):
+        """Limit is enforced via LIMIT in the Cypher query, not Python slicing."""
         instance = _make_instance()
         instance._exec_cypher = MagicMock(return_value=[
-            {"source": f"n{i}", "relationship": "R", "target": f"m{i}"} for i in range(10)
+            {"source": "n0", "relationship": "R", "target": "m0"},
         ])
-        results = instance.get_all({"user_id": "u1"}, limit=3)
-        assert len(results) == 3
+        instance.get_all({"user_id": "u1"}, limit=3)
+        # Verify limit was passed as a parameter to the query
+        cypher_stmt = instance._exec_cypher.call_args[0][0]
+        assert "LIMIT %s" in cypher_stmt
+        params = instance._exec_cypher.call_args[1].get("params") or instance._exec_cypher.call_args[0][2]
+        assert 3 in params
 
 
 class TestAdd: