Add TransitionHasDestineeIfIsSentExternal validator

This commit introduces a new validator to ensure that transitions marked as 'sent' have a designated external recipient. It includes related tests for scenarios with and without recipients and covers integration with the workflow context.

src/Bundle/ChillMainBundle/Form/WorkflowStepType.php

@@ -86,7 +86,6 @@ class WorkflowStepType extends AbstractType
         $builder
             ->add('transition', ChoiceType::class, [
                 'label' => 'workflow.Next step',
-                'mapped' => false,
                 'multiple' => false,
                 'expanded' => true,
                 'choices' => $choices,