Fix is_async status of agent execution task (opensearch-project#3960) (opensearch-project#3962)

(cherry picked from commit 23b98cc)

Co-authored-by: Rithin Pullela <rithinp@amazon.com>

Author: opensearch-trigger-bot[bot]

common/src/main/java/org/opensearch/ml/common/MLTask.java

@@ -73,6 +73,7 @@ public class MLTask implements ToXContentObject, Writeable {
     @Setter
     private String error;
     private User user; // TODO: support document level access control later
+    @Setter
     private boolean async;
     @Setter
     private Map<String, Object> remoteJob;

ml-algorithms/src/main/java/org/opensearch/ml/engine/algorithms/agent/MLAgentExecutor.java

@@ -407,6 +407,7 @@ private void executeAgent(
                 agentResponse.put(PARENT_INTERACTION_ID, parentInteractionId);
             }
             mlTask.setResponse(agentResponse);
+            mlTask.setAsync(true);
 
             indexMLTask(mlTask, ActionListener.wrap(indexResponse -> {
                 String taskId = indexResponse.getId();