Catalog search no longer breaks on a trailing operator

A merged pull request fixes a long-standing catalog search bug: MySQL error 1064 on a trailing operator.

The search-suggest request sends the term while the shopper is still typing, so a search like test - reached the database with a bare trailing -. In MySQL boolean mode that is invalid syntax and the query failed with error 1064.

What changed

prepareTerms() used to copy every standalone operator and bracket straight into the boolean-mode query, so test - produced "test" -. The operator support was never actually usable anyway: an operator only worked as a separate word (-brush was treated as a phrase), test* never truncated, and PostgreSQL, SQLite and the LIKE search ignored operators altogether.

Now each word is wrapped in double quotes and any word made up only of operator or bracket characters is dropped. The result is predictable: test - brush finds products matching test or brush, and no longer silently excludes brush. On SQLite the same words are dropped, so test - no longer produces a LIKE value that matches nothing, and that value no longer starts with a stray space.

Two related limits were also made consistent:

• max_query_words previously limited only the terms, not the words bound to the :query parameter, so the full-text query had no word limit at all. It now has one on every database.
• min_query_length applied only to the whole search text. Each word shorter than the minimum is now dropped, on every database and for both the full-text query and the LIKE conditions. A LIKE search left with no word finds no product, and the “Maximum words count” notice counts the same words the search actually uses.

A search for only operator characters such as --- used to match every product on SQLite; it now matches nothing, matching MySQL behaviour.