Notes from task breakdown:

• create a new statement serializer class that wraps the existing one and ensures the required fields are contained in the serialization
• use new serializer and adjust statement spec to mark the listed fields as required
  • use new serializer in GetItemStatementJsonPresenter
  • use new serializer in ItemDataSerializer wrapped in StatementListSerializer (which we need to instantiate ourselves)
  • use new serializer in GetItemStatementsJsonPresenter wrapped in StatementListSerializer (which we need to instantiate ourselves)

This can now be verified on beta:

• item: https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/entities/items/Q11
• statements: https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/entities/items/Q11/statements
• single statement: https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/statements/Q11$569c11d8-4564-5981-80e7-189c3841ae3c
• single statement on item: https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/entities/items/Q11/statements/Q11$569c11d8-4564-5981-80e7-189c3841ae3c

I moved this back to ToDo for now, since I noticed something in the task description, which we had actually missed: T309633: 🟦️ Adjust examples in the OpenAPI specification to include all required statement fields

Responses and examples look good - verified.