fix: switch built-in-agent to single-route mode for smoke probe compatibility

Author: jpr5

showcase/integrations/built-in-agent/src/app/api/copilotkit/route.ts

@@ -13,17 +13,9 @@ const runtime = new CopilotRuntime({
 const handler = createCopilotRuntimeHandler({
   runtime,
   basePath: "/api/copilotkit",
+  mode: "single-route",
 });
 
-async function withProbeCompat(req: Request): Promise<Response> {
-  const res = await handler(req);
-  if (res.status === 404) {
-    const body = await res.text();
-    return new Response(body, { status: 400, headers: res.headers });
-  }
-  return res;
-}
-
 export const GET = (req: Request) => handler(req);
-export const POST = (req: Request) => withProbeCompat(req);
+export const POST = (req: Request) => handler(req);
 export const OPTIONS = (req: Request) => handler(req);