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)

memory/utils/highlight_utils.py

@@ -0,0 +1,89 @@
+#
+#  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.
+#
+
+"""Highlight helpers for search results (wraps keywords in <em>)."""
+
+import re
+from collections.abc import Callable
+
+
+def highlight_text(
+    txt: str,
+    keywords: list[str],
+    is_english_fn: Callable[[str], bool] | None = None,
+) -> str:
+    """Wrap keyword matches in text with <em>, by sentence.
+
+    - If is_english_fn(sentence) is True: use word-boundary regex.
+    - Otherwise: literal replace (longest keywords first).
+    Only sentences that contain a match are included.
+    """
+    if not txt or not keywords:
+        return ""
+
+    txt = re.sub(r"[\r\n]", " ", txt, flags=re.IGNORECASE | re.MULTILINE)
+    txt_list = []
+
+    for t in re.split(r"[.?!;\n]", txt):
+        t = t.strip()
+        if not t:
+            continue
+
+        if is_english_fn is None or is_english_fn(t):
+            for w in keywords:
+                t = re.sub(
+                    r"(^|[ .?/'\"\(\)!,:;-])(%s)([ .?/'\"\(\)!,:;-]|$)" % re.escape(w),
+                    r"\1<em>\2</em>\3",
+                    t,
+                    flags=re.IGNORECASE | re.MULTILINE,
+                )
+        else:
+            for w in sorted(keywords, key=len, reverse=True):
+                t = re.sub(
+                    re.escape(w),
+                    f"<em>{w}</em>",
+                    t,
+                    flags=re.IGNORECASE | re.MULTILINE,
+                )
+
+        if re.search(r"<em>[^<>]+</em>", t, flags=re.IGNORECASE | re.MULTILINE):
+            txt_list.append(t)
+
+    return "...".join(txt_list) if txt_list else txt
+
+
+def get_highlight_from_messages(
+    messages: list[dict] | None,
+    keywords: list[str],
+    field_name: str,
+    is_english_fn: Callable[[str], bool] | None = None,
+) -> dict[str, str]:
+    """Build id -> highlighted text from a list of message dicts."""
+    if not messages or not keywords:
+        return {}
+
+    ans = {}
+    for doc in messages:
+        doc_id = doc.get("id")
+        if not doc_id:
+            continue
+        txt = doc.get(field_name)
+        if not txt or not isinstance(txt, str):
+            continue
+        highlighted = highlight_text(txt, keywords, is_english_fn)
+        if highlighted and re.search(r"<em>[^<>]+</em>", highlighted, flags=re.IGNORECASE | re.MULTILINE):
+            ans[doc_id] = highlighted
+    return ans