fix: runAsync handles the async response of sessionService.appendEvent

PiperOrigin-RevId: 798203450

Author: google-genai-bot

core/src/main/java/com/google/adk/runner/Runner.java

@@ -113,7 +113,7 @@ public BaseSessionService sessionService() {
    *
    * @throws IllegalArgumentException if message has no parts.
    */
-  private void appendNewMessageToSession(
+  private Single<Event> appendNewMessageToSession(
       Session session,
       Content newMessage,
       InvocationContext invocationContext,
@@ -153,7 +153,7 @@ private void appendNewMessageToSession(
             .author("user")
             .content(Optional.of(newMessage))
             .build();
-    this.sessionService.appendEvent(session, event);
+    return this.sessionService.appendEvent(session, event);
   }
 
   /**
@@ -217,14 +217,24 @@ public Flowable<Event> runAsync(Session session, Content newMessage, RunConfig r
                         /* liveRequestQueue= */ Optional.empty(),
                         runConfig);
 
-                if (newMessage != null) {
-                  appendNewMessageToSession(
-                      sess, newMessage, invocationContext, runConfig.saveInputBlobsAsArtifacts());
-                }
-
-                invocationContext.agent(this.findAgentToRun(sess, rootAgent));
-                Flowable<Event> events = invocationContext.agent().runAsync(invocationContext);
-                return events.doOnNext(event -> this.sessionService.appendEvent(sess, event));
+                Single<Event> singleEvent =
+                    (newMessage != null)
+                        ? appendNewMessageToSession(
+                            sess,
+                            newMessage,
+                            invocationContext,
+                            runConfig.saveInputBlobsAsArtifacts())
+                        : Single.just(null);
+                return singleEvent.flatMapPublisher(
+                    ignored -> {
+                      invocationContext.agent(this.findAgentToRun(sess, rootAgent));
+                      return invocationContext
+                          .agent()
+                          .runAsync(invocationContext)
+                          .flatMap(
+                              agentEvent ->
+                                  this.sessionService.appendEvent(sess, agentEvent).toFlowable());
+                    });
               })
           .doOnError(
               throwable -> {