The Gemini Flash image path (`generateContent`) dropped both `aspect_ratio`
and `image_size`: `generate()` never forwarded them and
`_generate_gemini_flash` did not accept them, so every request fell back to
1:1 / input-matched output. The Imagen path was unaffected.
Forward the hints and emit them under
`generationConfig.responseFormat.image` per the current Gemini API. Aspect
ratio is validated against the accepted set; `imageSize` is validated against
{512,1K,2K,4K} and only sent to Gemini 3+ image models, since
`gemini-2.5-flash-image` supports only `aspectRatio`.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
4986590bd7