Feature: match CJK terms through their bigram fields in place (#14156)

QUERY-mode searches blended a separate bigram clause in at the top of the
query, built from the parsed AST's free-text tokens. Because it sat beside
the exact clause rather than inside the query, nothing around a CJK term
constrained its bigram match: an exclusion that was one OR branch's own
condition could never reach it, so "(東京 AND NOT secret) OR bill" still
returned the secret document.

Widen each CJK leaf where it sits instead, through emit()'s rewrite_leaf
hook, so every AND, NOT, REQUIRE, boost and field restriction around the
leaf applies to its bigram match too. Negated leaves are widened on
purpose, so "NOT X" excludes exactly what "X" matches.

src/documents/tests/search/test_query.py

@@ -522,8 +522,8 @@ class TestEmitErrorContract:
         THEN:
             - The emit failure is caught and mapped to a SearchQueryError
               (400) rather than propagating as a raw QueryError. This
-              fails at the whole-tree "exact" clause emission
-              (`exact = tantivy_emit(result.ast, ...)`), the same path
+              fails at the whole-tree "exact" clause emission (the
+              `tantivy_emit` call in parse_user_query), the same path
               TestRealQueriesRouteCorrectly::test_text_range_is_a_400_naming_the_field
               already covers without the NOT wrapper: `_negation_clauses`
               never runs here, since the whole-tree emit already raises