Update base_push_notification_sender.py

- Honor PushNotificationConfig.authentication field in BasePushNotificationSender
- Add Authorization header when Bearer scheme is configured
- Maintain backward compatibility with existing X-A2A-Notification-Token header

Author: Tareqox

src/a2a/server/tasks/base_push_notification_sender.py

@@ -52,13 +52,21 @@ async def _dispatch_notification(
     ) -> bool:
         url = push_info.url
         try:
-            headers = None
+            headers = {}
             if push_info.token:
-                headers = {'X-A2A-Notification-Token': push_info.token}
+                headers['X-A2A-Notification-Token'] = push_info.token
+            
+            # Add authentication header if configured
+            if push_info.authentication and push_info.authentication.schemes:
+                for scheme in push_info.authentication.schemes:
+                    if scheme.lower() == 'bearer' and push_info.authentication.credentials:
+                        headers['Authorization'] = f'Bearer {push_info.authentication.credentials}'
+                        break
+            
             response = await self._client.post(
                 url,
                 json=task.model_dump(mode='json', exclude_none=True),
-                headers=headers,
+                headers=headers if headers else None,
             )
             response.raise_for_status()
             logger.info(