fix: use mb_strlen in substrReplace for multibyte safety

refs: #57838

Author: artengin

src/Illuminate/Support/Str.php

@@ -1748,7 +1748,7 @@ public static function substrCount($haystack, $needle, $offset = 0, $length = nu
     public static function substrReplace($string, $replace, $offset = 0, $length = null)
     {
         if ($length === null) {
-            $length = strlen($string);
+            $length = mb_strlen($string);
         }
 
         return substr_replace($string, $replace, $offset, $length);