fix: truncate URI Template prefix modifier by character, not byte

Operator::inject() applied the ":N" prefix modifier with the
byte-oriented substr(), splitting multibyte UTF-8 characters in half
and producing invalid percent-encoded output (e.g. the euro sign
truncated to its first byte only).

RFC 6570 Section 2.4.1 defines the prefix length as a count of
characters, so use mb_substr() with an explicit UTF-8 encoding
instead. ext-mbstring is already a hard dependency of this package.

uri/UriTemplate/Operator.php

@@ -19,10 +19,10 @@ use Stringable;
 
 use function implode;
 use function is_array;
+use function mb_substr;
 use function preg_match;
 use function rawurlencode;
 use function str_contains;
-use function substr;
 
 /**
  * Processing behavior according to the expression type operator.
@@ -156,7 +156,9 @@ enum Operator: string
         }
 
         if (':' === $varSpec->modifier) {
-            $value = substr($value, 0, $varSpec->position);
+            // RFC 6570 Section 2.4.1: the prefix length is a count of characters, not octets,
+            // so a multibyte-aware truncation is required to avoid splitting a UTF-8 character.
+            $value = mb_substr($value, 0, $varSpec->position, 'UTF-8');
         }
 
         return [$this->decode($value), $this->isNamed()];