fix: validate UTF-8 input before regex matching

Ensures invalid UTF-8 strings are sanitized before reaching preg_match
with the /u flag, preventing silent regex failures.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: deemonic

src/BlaspService.php

@@ -257,6 +257,10 @@ public function check(string $string): self
             throw new Exception('No string to check');
         }
 
+        if (!mb_check_encoding($string, 'UTF-8')) {
+            $string = mb_convert_encoding($string, 'UTF-8', 'UTF-8');
+        }
+
         $this->sourceString = $string;
 
         $this->cleanString = $string;