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/nonStreamingHandler.js

@@ -37,6 +37,12 @@ export function translateNonStreamingResponse(responseBody, targetFormat, source
             function: { name: part.functionCall.name, arguments: JSON.stringify(part.functionCall.args || {}) }
           });
         }
+        // Handle inline image data (from image generation models)
+        const inlineData = part.inlineData || part.inline_data;
+        if (inlineData?.data) {
+          const mimeType = inlineData.mimeType || inlineData.mime_type || "image/png";
+          textContent += `\n![image](data:${mimeType};base64,${inlineData.data})\n`;
+        }
       }
     }