fix error handling in default request handler

Author: sokoliva

src/a2a/server/request_handlers/default_request_handler_v2.py

@@ -340,7 +340,7 @@ async def on_create_task_push_notification_config(  # noqa: D102
         context: ServerCallContext,
     ) -> TaskPushNotificationConfig:
         if not self._push_config_store:
-            raise UnsupportedOperationError
+            raise PushNotificationNotSupportedError
 
         task_id = params.task_id
         task: Task | None = await self.task_store.get(task_id, context)
@@ -367,7 +367,7 @@ async def on_get_task_push_notification_config(  # noqa: D102
         context: ServerCallContext,
     ) -> TaskPushNotificationConfig:
         if not self._push_config_store:
-            raise UnsupportedOperationError
+            raise PushNotificationNotSupportedError
 
         task_id = params.task_id
         config_id = params.id