`JsonApi\ItemNormalizer::denormalize()` is called twice resulting in nulled input properties

[daFish]

API Platform version(s) affected: 4.2.17

Description
After PR #7779, AbstractItemNormalizer::supportsDenormalization() returns true for input DTOs during serializer re-entry (via the api_platform_input flag).
For JSON:API, this causes JsonApi\ItemNormalizer to claim the DTO and attempt to extract data.attributes from already-flat data, yielding an empty array.

The flow:

1. JsonApi\ItemNormalizer::denormalize() unwraps data.attributes → flat array
2. AbstractItemNormalizer detects input DTO → re-enters serializer with flat data
3. JsonApi\ItemNormalizer::supportsDenormalization() now returns true (new flag)
4. JsonApi\ItemNormalizer::denormalize() runs again on flat data → $data['data']['attributes'] is null → empty
5. Input DTO properties are all null