feat(antigravity): native image generation support

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>

open-sse/handlers/imageProviders/index.js

@@ -11,6 +11,7 @@ import stabilityAi from "./stabilityAi.js";
 import blackForestLabs from "./blackForestLabs.js";
 import runwayml from "./runwayml.js";
 import cloudflareAi from "./cloudflareAi.js";
+import antigravity from "./antigravity.js";
 
 const ADAPTERS = {
   openai: createOpenAIAdapter("openai"),
@@ -25,6 +26,7 @@ const ADAPTERS = {
   comfyui,
   huggingface,
   nanobanana,
+  antigravity,
   "fal-ai": falAi,
   "stability-ai": stabilityAi,
   "black-forest-labs": blackForestLabs,