Mirrors Meta's POST /{accountId}/adcreatives.

Body carries Meta's own ad-creative-create fields — an object_story_spec (single image/video link_data/video_data, carousel via child_attachments, catalog/DPA via template_data + product_set_id), or object_story_id + page_id to reuse an existing post, plus optional degrees_of_freedom_spec for GenAI creative features (text generation, image uncrop, background generation) — and is forwarded verbatim.

• Write — changes a live ad account immediately; needs a write-scoped token.