Problem

When Symfony's TypeInfo resolves array<T> (single generic parameter, no explicit key type), the key defaults to arrayKey() — a union(int, string). ArrayDescriber splits this into separate array<int, T> and array<string, T> types, producing an incorrect OpenAPI schema:

{
  "anyOf": [
    { "type": "array", "items": { "$ref": "#/components/schemas/MyDto" } },
    { "type": "object", "additionalProperties": { "$ref": "#/components/schemas/MyDto" } }
  ]
}

The correct output should be just { "type": "array", "items": { "$ref": "#/components/schemas/MyDto" } }, since array<T> in PHP — especially in the context of JSON API responses — means a sequential array (JSON array), not "either array or object".

Traversable objects as generic parameters

When a Traversable object is used as a generic parameter (e.g. list<MyIterableClass>), StringTypeResolver wraps it in CollectionType(ObjectType) with no key/value type info. ArrayDescriber then describes this as array<unknown> (because getCollectionValueType() returns mixed()), losing the type reference entirely.

The fix detects when the wrapped type is an ObjectType (meaning StringTypeResolver auto-wrapped a Traversable class) and delegates to the chain so ClassDescriber can create a proper $ref.

Reproduction

final class EventDTO
{
    /** @var RefereeDTO[] */
    public array $referees;
}

With type_info: true (Symfony ≥ 7.3), the referees property schema includes both array and object types instead of just array.

Relation to #2508

Issue #2508 reported two symptoms:

1. type_info: false: "items": {} (empty, no type info) — this was caused by ConstructorExtractor not reading @var on promoted constructor properties, and was fixed upstream in Symfony 7.4 ([Serialization] with_constructor_extractor: true - @var docs on promoted properties ignored symfony/symfony#60795).
2. type_info: true: "oneOf": [{ "type": "array" }, { "type": "object" }] — this is the ArrayDescriber issue fixed by this PR. Even with Symfony 7.4, once TypeInfo correctly reads the type, ArrayDescriber still splits the arrayKey() union into anyOf: [array, object].

This PR fixes the remaining symptom 2 from #2508.