[php-src] PR #25219: Fix stack overflow in mb_check_encoding() and mb_convert_encoding()

Pull Request: https://github.com/php/php-src/pull/25219 Author: eyupcanakman mb_check_encoding() and mb_convert_encoding() walk array arguments with php_mb_check_encoding_recursive() and php_mb_convert_encoding_recursive(). Neither helper looks at the stack limit before going one level deeper, so a deep enough array crashes the process. mb_convert_encoding() already segfaults at 30000 levels with the default stack. mbstring.c has had mb_check_stack_limit() since GH-20836, but only mb_convert_variables() uses it. Both helpers now call it and return early once the Error is thrown, so the walk stops there and a second nested array never adds a chained Error. The helper moves up the file so the convert path can see it. There is no GitHub issue for this.