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/services/usage/google.js

@@ -171,6 +171,9 @@ export async function getAntigravityUsage(accessToken, providerSpecificData, pro
         'claude-opus-4-6-thinking',
         'gpt-oss-120b-medium',
         'gemini-3-flash',
+        // Image generation models
+        'gemini-3.1-flash-image',
+        'gemini-3-pro-image',
       ];
 
       for (const [modelKey, info] of Object.entries(data.models)) {