Feat(memory): implement get_aggregation for OceanBase memory (#13428)

### What problem does this PR solve?

- Add aggregation_utils.aggregate_by_field for pure aggregation logic
- Wire OBConnection.get_aggregation to use it (unwrap tuple, pass
messages)
- Add unit tests for aggregate_by_field (no DB/heavy deps)

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

test/unit_test/memory/utils/test_ob_conn_aggregation.py

@@ -0,0 +1,55 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use it except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+"""Unit tests for OceanBase memory aggregation.
+
+Tests the pure aggregation logic used by OBConnection.get_aggregation,
+without requiring a real OceanBase instance or heavy dependencies.
+"""
+
+from memory.utils.aggregation_utils import aggregate_by_field
+
+
+class TestAggregateByField:
+    """Tests for aggregate_by_field (used by get_aggregation)."""
+
+    def test_empty_messages_returns_empty_list(self):
+        assert aggregate_by_field([], "message_type_kwd") == []
+        assert aggregate_by_field(None, "message_type_kwd") == []
+
+    def test_aggregates_field_values(self):
+        messages = [
+            {"id": "m1", "message_type_kwd": "user", "content_ltks": "a", "message_id": "msg1", "memory_id": "mem1", "status_int": 1},
+            {"id": "m2", "message_type_kwd": "assistant", "content_ltks": "b", "message_id": "msg2", "memory_id": "mem1", "status_int": 1},
+            {"id": "m3", "message_type_kwd": "user", "content_ltks": "c", "message_id": "msg3", "memory_id": "mem1", "status_int": 1},
+        ]
+        out = aggregate_by_field(messages, "message_type_kwd")
+        assert set(out) == {("user", 2), ("assistant", 1)}
+
+    def test_single_doc_result(self):
+        messages = [
+            {"id": "m1", "message_type_kwd": "user", "content_ltks": "x", "message_id": "msg1", "memory_id": "mem1", "status_int": 1}
+        ]
+        out = aggregate_by_field(messages, "message_type_kwd")
+        assert out == [("user", 1)]
+
+    def test_pre_aggregated_value_count_rows(self):
+        messages = [
+            {"value": "user", "count": 2},
+            {"value": "assistant", "count": 1},
+        ]
+        out = aggregate_by_field(messages, "message_type_kwd")
+        assert set(out) == {("user", 2), ("assistant", 1)}