gpt-image-2-text-to-imagetext-to-image4 paramsSlug has no vendor prefix despite the market/gpt/ doc path. No documented default on aspect_ratio, resolution or background — the prose says aspect_ratio "is set to auto by default" but the schema states none, so nothing is sent unless chosen. Five ratios are 1K-only here; the image-to-image sibling restricts only two.