Add unique constraint for PersonIdentifier, implement UniqueIdentifierConstraint with validation logic, and include supporting tests

- Introduce `UniqueIdentifierConstraint` and its validator for ensuring identifier uniqueness.
- Add a database-level unique constraint on `PersonIdentifier` (`definition_id`, `value`).
- Implement repository method to fetch identifiers by definition and value.
- Include integration and unit tests for validation and repository functionality.
- Update `Person` entity with `Assert\Valid` annotation for `identifiers`.

src/Bundle/ChillPersonBundle/Entity/Person.php

@@ -275,6 +275,7 @@ class Person implements HasCenterInterface, TrackCreationInterface, TrackUpdateI
 
     #[ORM\OneToMany(mappedBy: 'person', targetEntity: PersonIdentifier::class, cascade: ['persist', 'remove'], orphanRemoval: true)]
     #[RequiredIdentifierConstraint]
+    #[Assert\Valid]
     private Collection $identifiers;
 
     /**