feat : add Serializer::users() for collaborators

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Mcp/Tool/Serializer.php

@@ -134,6 +134,19 @@ final class Serializer
         ];
     }
 
+    /**
+     * @param Collection<int, User> $users
+     *
+     * @return list<array{id: ?int, username: ?string}>
+     */
+    public static function users(Collection $users): array
+    {
+        return $users->map(fn (User $u) => [
+            'id'       => $u->getId(),
+            'username' => $u->getUsername(),
+        ])->toArray();
+    }
+
     /**
      * @return null|array{id: ?int, title: ?string, color: ?string}
      */