feat: refactor callback validators to dedicated class

src/Validator/Constraints/Participant.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Validator\Constraints;
+
+use Symfony\Component\Validator\Constraint;
+
+#[\Attribute]
+class Participant extends Constraint
+{
+    public function getTargets(): array|string
+    {
+        return static::CLASS_CONSTRAINT;
+    }
+}