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/providers/registry/antigravity.js

@@ -18,6 +18,7 @@ export default {
     deprecationNotice: "RISK_NOTICE",
   },
   category: "oauth",
+  serviceKinds: ["llm", "image"],
   transport: {
     baseUrls: [
       "https://daily-cloudcode-pa.googleapis.com",
@@ -53,6 +54,9 @@ export default {
     { id: "claude-opus-4-6-thinking", name: "Claude Opus 4.6 (Thinking)" },
     { id: "gpt-oss-120b-medium", name: "GPT-OSS 120B (Medium)" },
     { id: "gemini-3-flash", name: "Gemini 3 Flash", thinking: false },
+    // Image generation models
+    { id: "gemini-3.1-flash-image", name: "Gemini 3.1 Flash (Image)", imageGen: true, capabilities: ["textToImage"] },
+    { id: "gemini-3-pro-image", name: "Gemini 3 Pro (Image)", imageGen: true, capabilities: ["textToImage"] },
   ],
   oauth: {
     authorizeUrl: "https://accounts.google.com/o/oauth2/v2/auth",