feat: trigram FTS5 index for CJK search, replace LIKE fallback (#16651)

* fix: bypass FTS5 for CJK queries in session_search

FTS5 default tokenizer splits CJK characters into individual tokens,
so multi-character queries like "大别山项目" become AND of single chars.
This produces few/no results compared to LIKE substring search.

For CJK queries, skip FTS5 entirely and use LIKE for accurate
phrase matching.

Fixes NousResearch/hermes-agent#15500

* fix: cache _contains_cjk, escape LIKE wildcards, add regression tests

On top of the CJK FTS5 bypass from #15509:

- Cache _contains_cjk() result in a local var to avoid redundant O(n)
  scans on every CJK query
- Escape %, _ in LIKE queries so literal wildcards in user input are
  not treated as SQL wildcards (consistent with other LIKE queries in
  hermes_state.py that use ESCAPE '\')
- Fix misleading comment ('or CJK fallback' → accurate description)
- Add 3 regression tests:
  - test_cjk_partial_fts5_results_supplemented_by_like (#15500 / #14829)
  - test_cjk_like_dedup_no_duplicates
  - test_cjk_like_escapes_wildcards (new wildcard escaping)

* feat: trigram FTS5 index for CJK search, replace LIKE fallback

Replace the LIKE '%query%' full-table-scan fallback for CJK queries with
a proper trigram FTS5 index (messages_fts_trigram).  The trigram tokenizer
creates overlapping 3-byte sequences so substring matching works natively
for any script — CJK, Thai, etc.

For queries with 3+ CJK characters: uses the trigram FTS5 table with
proper ranking, snippets, and indexed lookups.  For shorter queries
(1-2 CJK chars): falls back to LIKE since the trigram tokenizer needs
≥9 UTF-8 bytes (3 CJK chars) minimum.

Schema v10 migration creates the trigram table and backfills existing
messages.  Triggers keep the index in sync on INSERT/UPDATE/DELETE.

Builds on top of #16276 (bypass FTS5 for CJK, escape LIKE wildcards).

---------

Co-authored-by: vominh1919 <vominh1919@gmail.com>

tests/test_hermes_state.py

@@ -772,6 +772,51 @@ class TestCJKSearchFallback:
         results = db.search_messages("Agent通信")
         assert len(results) == 1
 
+    def test_cjk_partial_fts5_results_supplemented_by_like(self, db):
+        """When FTS5 returns *some* CJK results, LIKE must still find all matches.
+
+        Regression test for #15500 / #14829: FTS5 unicode61 tokenizer drops
+        certain CJK characters, so multi-character queries may return partial
+        results.  The LIKE path must always run for CJK queries.
+        """
+        db.create_session(session_id="s1", source="cli")
+        db.create_session(session_id="s2", source="telegram")
+        db.append_message("s1", role="user", content="昨晚讨论了记忆系统")
+        db.append_message("s2", role="user", content="昨晚的会议纪要已发送")
+        results = db.search_messages("昨晚")
+        assert len(results) == 2
+        session_ids = {r["session_id"] for r in results}
+        assert session_ids == {"s1", "s2"}
+
+    def test_cjk_like_dedup_no_duplicates(self, db):
+        """When FTS5 and LIKE both find the same message, no duplicates."""
+        db.create_session(session_id="s1", source="cli")
+        db.append_message("s1", role="user", content="测试去重逻辑")
+        results = db.search_messages("测试")
+        assert len(results) == 1
+
+    def test_cjk_like_escapes_wildcards(self, db):
+        """Special characters (%, _) in CJK queries are treated as literals."""
+        db.create_session(session_id="s1", source="cli")
+        db.create_session(session_id="s2", source="cli")
+        db.append_message("s1", role="user", content="达成100%完成率")
+        db.append_message("s2", role="user", content="达成100完成率是目标")
+        # The % in the query must be literal — should only match s1
+        results = db.search_messages("100%完成")
+        assert len(results) == 1
+        assert results[0]["session_id"] == "s1"
+
+    def test_cjk_trigram_preserves_boolean_operators(self, db):
+        """Boolean operators (OR, AND, NOT) work in CJK trigram queries."""
+        db.create_session(session_id="s1", source="cli")
+        db.create_session(session_id="s2", source="cli")
+        db.append_message("s1", role="user", content="记忆系统很好用")
+        db.append_message("s2", role="user", content="断裂连接需要修复")
+        results = db.search_messages("记忆系统 OR 断裂连接")
+        assert len(results) == 2
+        session_ids = {r["session_id"] for r in results}
+        assert session_ids == {"s1", "s2"}
+
 
 # =========================================================================
 # Session search and listing
@@ -1229,7 +1274,7 @@ class TestSchemaInit:
     def test_schema_version(self, db):
         cursor = db._conn.execute("SELECT version FROM schema_version")
         version = cursor.fetchone()[0]
-        assert version == 9
+        assert version == 10
 
     def test_title_column_exists(self, db):
         """Verify the title column was created in the sessions table."""
@@ -1290,7 +1335,7 @@ class TestSchemaInit:
 
         # Verify migration
         cursor = migrated_db._conn.execute("SELECT version FROM schema_version")
-        assert cursor.fetchone()[0] == 9
+        assert cursor.fetchone()[0] == 10
 
         # Verify title column exists and is NULL for existing sessions
         session = migrated_db.get_session("existing")