Generate an AI image from a text prompt using Google Gemini 3.1 Flash Image. Returns base64-encoded PNG. Optional aspectRatio (1:1, 16:9, 9:16, 3:2, 2:3, 4:3, 3:4, 4:5, 5:4, 21:9). Backed by Google's Gemini image preview model — fast, high-quality, supports text rendering inside images.