minor #18468 [Form][Validator] Add new unique entity validation on form type (94noni)

This PR was submitted for the 6.3 branch but it was squashed and merged into the 5.4 branch instead.

Discussion
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[Form][Validator] Add new unique entity validation on form type

Hello,

Where we have entity without validation on it, and want validation on the form type, this can be valuable I think

>**Note**I did not found such info on the doc so after slack discuss, I got the answer and thus proposed this doc

Commits
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fc94456 [Form][Validator] Add new unique entity validation on form type

Author: javiereguiluz

reference/constraints/UniqueEntity.rst

@@ -126,6 +126,29 @@ between all of the rows in your user table:
             }
         }
 
+        // src/Form/Type/UserType.php
+        namespace App\Form\Type;
+
+        // ...
+        // DON'T forget the following use statement!!!
+        use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
+
+        class UserType extends AbstractType
+        {
+            // ...
+
+            public function configureOptions(OptionsResolver $resolver): void
+            {
+                $resolver->setDefaults([
+                    // ...
+                    'data_class' => User::class,
+                    'constraints' => [
+                        new UniqueEntity(fields: ['email']),
+                    ],
+                ]);
+            }
+        }
+
 .. caution::
 
     This constraint doesn't provide any protection against `race conditions`_.