fix: unescape JSON string escape sequences in JSON_EXTRACT and ->> operator
JSON_EXTRACT and the ->> operator now properly unescape JSON string
escape sequences (\", \\, \/, \n, \t, \r, \b, \f, \uXXXX) instead of
returning them verbatim.
The fix calls unescape_string() in json_string_to_db_type() after
removing outer quotes from TEXT elements, matching SQLite behavior
where json_extract returns "the dequoted text for a JSON string value".
Closes #5751
Co-authored-by: Mikaël Francoeur <LeMikaelF@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>