test(graphql): cover doctrine collection in dto cursor pagination

[soyuka]

Summary

Adds a functional GraphQL regression test pinning the resolution of a
\Doctrine\Common\Collections\Collection field on a DTO returned through
the cursor-based pagination wrapper (edges/node).

The bug reported in #7038 (GraphQL operations return null for DTO collections while REST operations work correctly) is already fixed by upstream
commit b714a4451 ("fix(graphql): nested resources without graphqloperations
propagate fields", PR #8076).

Reverting that commit reproduces the exact error from the issue:
Cannot return null for non-nullable field "Category.name". The reporter's
scenario used Collection<Category> while #8076's test pinned array<Category>.
Both shapes hit the same path
(AbstractItemNormalizer::getAttributeValue → normalizeCollectionOfRelations
returns a plain array before the GraphQL field resolver wraps it in
ArrayPaginator), but only the array form was pinned by a test until now.

Reproduction

Pre-#8076: Cannot return null for non-nullable field "Category.name" on a GraphQL
query of a DTO that exposes a Collection<NestedResource>. Post-#8076: resolves
correctly. This test pins the Doctrine-Collection case to keep it from
regressing.

Test plan

• Added tests/Functional/GraphQl/Issue7038Test.php covering the
  Doctrine Collection<> field on a DTO under cursor pagination.
• All 153 GraphQl functional tests pass locally (8 new assertions).
• phpstan + cs-fixer clean.

Fixes #7038

[soyuka]

Closing per maintainer: bug already fixed by upstream commit b714a44 (#8076), no separate regression test needed.