fix(graphql): return identifier-only node on circular reference

[soyuka]

Summary

When a Doctrine entity has mutual ManyToOne/OneToMany references (e.g. Customer ↔ Address) and a GraphQL query traverses the cycle, the default Symfony circular_reference_handler returns the visited object's IRI as a string. The GraphQL ItemNormalizer and ObjectNormalizer then asserted the inner result was an array and threw Expected data to be an array., crashing the response with a 500.

This PR makes both normalizers return an identifier-only node (#itemResourceClass + #itemIdentifiers) when the inner normalize() yields a string, so the GraphQL field shape is preserved and the cycle is reported as already-visited instead of crashing.

Fixes #8080.

Test plan

• tests/Functional/GraphQl/Issue8080Test.php — minimal reproduction with two ApiResource entities (Issue8080Customer, Issue8080Address) cross-referencing each other; query traverses the cycle and asserts no errors key in the response.
• vendor/bin/phpunit tests/Functional/GraphQl — 148 tests, all green.
• vendor/bin/phpunit src/GraphQl/Tests — 242 tests, all green.
• vendor/bin/phpunit tests/Functional — 1564 tests, all green.
• vendor/bin/phpstan analyse src/GraphQl/Serializer/ItemNormalizer.php src/GraphQl/Serializer/ObjectNormalizer.php — no errors.