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()];

uri/UriTemplateTest.php

@@ -261,6 +261,20 @@ final class UriTemplateTest extends TestCase
         self::assertSame($expectedUri, $uriTemplate->expandToUrlOrFail($variables)->toAsciiString());
     }
 
+    public function testPrefixModifierTruncatesByCharacterNotByte(): void
+    {
+        // RFC 6570 Section 2.4.1: the prefix length counts characters, not octets, so a
+        // multibyte character must never be split. The euro sign and the musical G-clef
+        // are 3 and 4 bytes in UTF-8, respectively, but each is a single character.
+        $uriTemplate = new UriTemplate('{?currency:1}{clef:1}');
+        $variables = [
+            'currency' => "\u{20AC}uro",
+            'clef' => "\u{1D11E}stave",
+        ];
+
+        self::assertSame('?currency=%E2%82%AC%F0%9D%84%9E', $uriTemplate->expand($variables)->toString());
+    }
+
     public function testDisallowNestedArrayExpansion(): void
     {
         $template = 'http://example.com{?query,data*,foo*}';