feat(memory): implement get_highlight for OceanBase memory (#13449)

### What problem does this PR solve?

### Type of change

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

test/unit_test/memory/utils/test_ob_conn_highlight.py

@@ -0,0 +1,79 @@
+#
+#  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 get_highlight.
+
+Tests the pure highlight logic used by OBConnection.get_highlight,
+without requiring a real OceanBase instance or heavy dependencies.
+"""
+
+from memory.utils.highlight_utils import get_highlight_from_messages, highlight_text
+
+
+class TestHighlightText:
+    """Tests for highlight_text (word-boundary mode when is_english_fn is None)."""
+
+    def test_empty_text_returns_empty(self):
+        assert highlight_text("", ["foo"]) == ""
+        assert highlight_text("hello", []) == ""
+
+    def test_wraps_keyword_with_em(self):
+        out = highlight_text("The quick brown fox.", ["quick"], None)
+        assert "<em>quick</em>" in out
+        assert "The" in out and "brown fox" in out
+
+    def test_only_sentences_with_match_included(self):
+        out = highlight_text(
+            "First sentence. Second has keyword. Third none.",
+            ["keyword"],
+            None,
+        )
+        assert "Second has <em>keyword</em>" in out
+        assert "First sentence" not in out and "Third none" not in out
+
+    def test_multiple_keywords(self):
+        out = highlight_text("Alpha and beta here.", ["Alpha", "beta"], None)
+        assert "<em>Alpha</em>" in out and "<em>beta</em>" in out
+
+
+class TestGetHighlightFromMessages:
+    """Tests for get_highlight_from_messages (used by get_highlight)."""
+
+    def test_empty_messages_returns_empty_dict(self):
+        assert get_highlight_from_messages([], ["k"], "content_ltks") == {}
+        assert get_highlight_from_messages(None, ["k"], "content_ltks") == {}
+
+    def test_empty_keywords_returns_empty_dict(self):
+        assert get_highlight_from_messages(
+            [{"id": "m1", "content_ltks": "hello"}], [], "content_ltks"
+        ) == {}
+
+    def test_returns_id_to_highlighted_text(self):
+        messages = [
+            {"id": "msg1", "content_ltks": "The cat sat."},
+            {"id": "msg2", "content_ltks": "The dog ran."},
+        ]
+        out = get_highlight_from_messages(messages, ["cat"], "content_ltks")
+        assert list(out.keys()) == ["msg1"]
+        assert "<em>cat</em>" in out["msg1"]
+        out2 = get_highlight_from_messages(messages, ["dog"], "content_ltks")
+        assert list(out2.keys()) == ["msg2"]
+        assert "<em>dog</em>" in out2["msg2"]
+
+    def test_skips_docs_without_field(self):
+        messages = [{"id": "m1"}, {"id": "m2", "content_ltks": "hello world."}]
+        out = get_highlight_from_messages(messages, ["hello"], "content_ltks")
+        assert "m2" in out and "<em>hello</em>" in out["m2"]