fix: validate mandatory services selection

src/Validator/Constraints/MandatoryAdditionalServicesSelected.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Validator\Constraints;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Validates that all mandatory additional services are selected.
+ *
+ * Applied on ParticipantEditDto to enforce mandatory additional services based
+ * on participant age and booking eligibility in both create and edit flows.
+ */
+#[\Attribute]
+class MandatoryAdditionalServicesSelected extends Constraint
+{
+    public string $message = 'Bitte wähle alle Pflichtleistungen aus.';
+
+    public function getTargets(): array|string
+    {
+        return static::CLASS_CONSTRAINT;
+    }
+}
+