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/chatCore.js

@@ -71,6 +71,13 @@ export async function handleChatCore({ body, modelInfo, credentials, log, onCred
   const providerRequiresStreaming = PROVIDERS[provider]?.forceStream === true;
   let stream = providerRequiresStreaming ? true : (body.stream !== false);
 
+  // Image generation models require non-streaming (Google v1internal:generateContent)
+  const modelType = getModelType(alias, model);
+  const isImageGenModel = modelType === "imageGen" || /image|imagen|image-generation/i.test(model);
+  if (isImageGenModel && (provider === "antigravity" || provider === "gemini-cli")) {
+    stream = false;
+  }
+
   // DeepSeek-TUI: interactive TUI panel sends stream:true and needs SSE.
   // Non-interactive mode (-p flag) sends without stream and can't parse SSE.
   // Only force non-streaming when client didn't explicitly request it.