Add image generation for Antigravity provider via gemini-3.1-flash-image
and gemini-3-pro-image, exposed through Text to Image UI and
/v1/images/generations.
- registry: serviceKinds ['llm','image'] + image model entries
- executor: image model detection + image_gen request envelope
- chatCore: force stream=false for image models (generateContent)
- nonStreamingHandler: parse inlineData -> markdown image
- imageGenerationCore: useExecutor fast-path for executor delegation
- imageProviders/antigravity: image adapter with image input support
- usage/google: image models in quota whitelist
Co-authored-by: Cursor <cursoragent@cursor.com>
