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/imageGenerationCore.js

@@ -50,6 +50,47 @@ export async function handleImageGenerationCore({
     );
   }
 
+  // Executor-delegating adapters: skip manual URL/headers/body, use the proven executor flow
+  if (adapter.useExecutor && adapter.executeViaExecutor) {
+    try {
+      log?.debug?.("IMAGE", `${provider.toUpperCase()} | ${model} | prompt="${body.prompt.slice(0, 50)}..." (executor)`);
+      const responseBody = await adapter.executeViaExecutor(model, body, credentials, log);
+      if (onRequestSuccess) await onRequestSuccess();
+      const normalized = adapter.normalize(responseBody, body.prompt);
+      const finalBody = (normalized.created && Array.isArray(normalized.data)) ? normalized : responseBody;
+
+      if (binaryOutput) {
+        const first = finalBody.data?.[0];
+        let b64 = first?.b64_json;
+        if (!b64 && first?.url) {
+          try { b64 = await urlToBase64(first.url); } catch {}
+        }
+        if (b64) {
+          const buf = Buffer.from(b64, "base64");
+          const fmt = (body.output_format || "png").toLowerCase();
+          const mime = fmt === "jpeg" || fmt === "jpg" ? "image/jpeg" : fmt === "webp" ? "image/webp" : "image/png";
+          return {
+            success: true,
+            response: new Response(buf, {
+              headers: { "Content-Type": mime, "Content-Disposition": `inline; filename="image.${fmt === "jpeg" ? "jpg" : fmt}"`, "Access-Control-Allow-Origin": "*" },
+            }),
+          };
+        }
+      }
+
+      return {
+        success: true,
+        response: new Response(JSON.stringify(finalBody), {
+          headers: { "Content-Type": "application/json", "Access-Control-Allow-Origin": "*" },
+        }),
+      };
+    } catch (error) {
+      const errMsg = formatProviderError(error, provider, model, HTTP_STATUS.BAD_GATEWAY);
+      log?.debug?.("IMAGE", `Executor error: ${errMsg}`);
+      return createErrorResult(HTTP_STATUS.BAD_GATEWAY, errMsg);
+    }
+  }
+
   let url;
   let headers;
   let requestBody;