Returns the most recent (up to 100) resources available to compose a message from. Use this to pick a resource_id before calling GET /messages/contents/{resource_id} (retrieveMessageContent) to render the final HTML. The content type depends on message_type: email-article returns articles from publication_id; email-promo and email-renewal return lift letters from promoted_publication_id. This endpoint does NOT apply to email-free-style — those messages have no resource and are composed from a template only via GET /templates (retrieveTemplates).