[Autocomplete] Fix LIKE ESCAPE clause breaking search on PostgreSQL

[Amoifr]

Q	A
Bug fix?	yes
New feature?	no
Tickets	Fix #3668
License	MIT

Since 2.36, EntitySearchUtil escapes the LIKE wildcards (%, _) in the
autocomplete search query and pairs that with an ESCAPE '\' clause so the
escaped wildcards are treated literally.

That backslash breaks on PostgreSQL: with standard_conforming_strings = off
(older setups / some PDO driver versions) the backslash escapes the closing
quote of the SQL string literal, producing invalid SQL. It surfaces as
SQLSTATE[HY093] Invalid parameter number or
SQLSTATE[22025] invalid escape string and makes every text search throw.

Fix

Use a backslash-free LIKE escape character (!) via the platform's
escapeStringForLike(). The explicit ESCAPE clause is kept on purpose:
SQLite has no default LIKE escape character, so dropping it would silently
re-open the wildcard-injection the 2.36 hardening closed.

Note: the workaround suggested in the issue (escapeStringForLike($q, '\') +
removing the ESCAPE clause) wouldn't fully work — escapeStringForLike($q, '\')
produces byte-identical output to the current addcslashes($q, '\\%_'), so the
only effective change there is dropping ESCAPE, which regresses wildcard
escaping on SQLite. Switching the escape character is what actually fixes
PostgreSQL without that regression.

Tests

• SQLite (CI): the existing testItEscapesLikeWildcardsInTheQuery is extended
  with a literal ! case and stays green — it can only pass if the ESCAPE
  clause works.
• PostgreSQL (verified locally against a real server): the old ESCAPE '\'
  errors with standard_conforming_strings=off; the new ESCAPE '!' works
  under both settings and correctly matches literal %, _ and !.