Fix JSON:API collection included schema

[Will-thom]

Fixes #7956.

Preserves the JSON:API included schema when building OpenAPI schemas for collection responses and adds a functional regression test.

Tests:

• php -l src/JsonApi/JsonSchema/SchemaFactory.php
• php -l tests/Functional/OpenApiTest.php
• vendor/bin/phpunit tests/Functional/OpenApiTest.php --filter testJsonApiCollectionSchemaDocumentsIncludedResources
• vendor/bin/phpunit tests/Functional/OpenApiTest.php --filter testRetrieveTheOpenApiDocumentation
• vendor/bin/phpunit tests/Functional/OpenApiTest.php --filter testHasSchemasForMultipleFormats
• vendor/bin/phpunit tests/Functional/PaginationDisabledTest.php --filter testSchemaCollectionJsonApi

[soyuka]

Could be simpler by mutating $properties['data'] in place instead of introducing a $collectionProperties intermediate. That way included (and any future top-level key returned by buildDefinitionPropertiesSchema) flows through automatically:

$properties['data'] = [
    'type' => 'array',
    'items' => $properties['data'],
];

$schema['description'] = "$definitionName collection.";
$schema['allOf'] = [
    ['$ref' => $prefix.(false === $operation->getPaginationEnabled() ? self::COLLECTION_BASE_SCHEMA_NAME_NO_PAGINATION : self::COLLECTION_BASE_SCHEMA_NAME)],
    ['type' => 'object', 'properties' => $properties],
];

Removes the explicit isset($properties['included']) branch and keeps the wrapper shape consistent with the item schema.

[soyuka]

Nit: buildDefinitionPropertiesSchema returns 'included' => [...] only when relationships exist (see L357-376), so it can never be null here — isset() is safe. If the suggestion above is applied this branch goes away anyway.