bug #28252 [DoctrineBridge] support __toString as documented for UniqueEntityValidator (dmaicher)

This PR was squashed before being merged into the 3.4 branch (closes #28252).

Discussion
----------

[DoctrineBridge] support __toString as documented for UniqueEntityValidator

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #28245
| License       | MIT
| Doc PR        | -

This fixes https://github.com/symfony/symfony/issues/28245.

It brings back handling `__toString` as documented for invalid values.

Commits
-------

2ac883a99b [DoctrineBridge] support __toString as documented for UniqueEntityValidator

src/Symfony/Bridge/Doctrine/Tests/Validator/Constraints/UniqueEntityValidatorTest.php

@@ -482,7 +482,7 @@ class UniqueEntityValidatorTest extends ConstraintValidatorTestCase
 
         $this->buildViolation('myMessage')
             ->atPath('property.path.single')
-            ->setParameter('{{ value }}', 'object("Symfony\Bridge\Doctrine\Tests\Fixtures\SingleIntIdEntity") identified by (id => 1)')
+            ->setParameter('{{ value }}', 'foo')
             ->setInvalidValue($entity1)
             ->setCode(UniqueEntity::NOT_UNIQUE_ERROR)
             ->setCause(array($associated, $associated2))

src/Symfony/Bridge/Doctrine/Validator/Constraints/UniqueEntityValidator.php

@@ -186,6 +186,10 @@ class UniqueEntityValidator extends ConstraintValidator
             return $this->formatValue($value, self::PRETTY_DATE);
         }
 
+        if (\method_exists($value, '__toString')) {
+            return (string) $value;
+        }
+
         if ($class->getName() !== $idClass = \get_class($value)) {
             // non unique value might be a composite PK that consists of other entity objects
             if ($em->getMetadataFactory()->hasMetadataFor($idClass)) {