fix(showcase): convert v2 runtime 404 to 400 for smoke probe compat

The v2 createCopilotRuntimeHandler returns 404 for malformed POSTs to
the base path, while the v1 API returns 400. The smoke probe treats
404 as "route not wired" and marks agent:built-in-agent red. Wrap the
POST handler to convert 404→400 so the probe reads it as proof-of-life.

Author: jpr5

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

@@ -15,6 +15,15 @@ const handler = createCopilotRuntimeHandler({
   basePath: "/api/copilotkit",
 });
 
+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) => handler(req);
+export const POST = (req: Request) => withProbeCompat(req);
 export const OPTIONS = (req: Request) => handler(req);