fix(search): support CJK terms in FTS5 BM25 - #401
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What does this PR do?
Fixes FTS5 BM25 keyword retrieval for CJK text. SQLite unicode61 treats a contiguous Chinese string as one token, so a passage containing 数据库检索系统 could not be found by a query for 数据库.
New BM25 databases now index CJK runs as unigrams and bigrams. CJK queries require all bigrams in each term, while non-CJK query behavior remains unchanged. The database stores its format in SQLite PRAGMA user_version, so existing BM25 artifacts continue to use their legacy query format after an upgrade.
Adds regression coverage for Chinese substring retrieval after reopening the database and for loading an existing-format database.
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