gpt-image-2-image-to-imageimage-to-image5 paramsSlug has no vendor prefix despite the market/gpt/ doc path. Only 5:4 and 4:5 are 1K-only here, where the text-to-image sibling also restricts 3:1, 1:3 and 9:21. No documented file-size limit on input_urls, unlike GPT Image 1.5.