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