fix(samples): filter out heartbeat messages from webhook stream UI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pengying

samples/frontend/src/components/WebhookStream.tsx

@@ -19,6 +19,7 @@ export default function WebhookStream() {
 
       es.onopen = () => setConnected(true)
       es.onmessage = (event) => {
+        if (event.data === 'heartbeat') return
         try {
           const data = JSON.parse(event.data)
           if (data.type === 'connected') return