Improve the SqlWriter API to make node grouping less error-prone

write_search_node_to_sql() has a comment about wrapping the SQLite for every multi-term search node in parentheses to generate the correct conditions:
https://github.com/ankitects/anki/blob/d80273ea71d4e9a3cab91b5a2e5906366809e3ea/rslib/src/search/sqlwriter.rs#L127-L129

It's easy to miss that though and introduce bugs as happened recently (#5500) and in the past when the API was introduced (https://github.com/ankitects/anki/pull/1754#issuecomment-1081282942) so it's worth looking into improving this.

Start in rslib/src/search/sqlwriter.rs at write_search_node_to_sql() and read the comment around lines 127-129. Review the grouping problem described in #5500 and the prior API discussion in pull request #1754. Done means the SqlWriter API makes it difficult to omit the parentheses needed for multi-term search nodes and avoids the reported class of SQL-condition bugs.

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