chore: Update Example to print the actual response returned from Agent (#203)

# Description
The current helloworld example only ends up printing `Response:
io.a2a.spec.SendMessageResponse@63252cf7` when it does
`System.out.println("Response: " + response.toString());`. I expect the
actual intent to be to print the response message sent by the Agent
which is what this update does.

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- [X] Ensure the tests pass
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- The current helloworld example READMEs (client and server) seem to
need work beyond this minor update. Happy to handle that under another
Issue/PR if strategic and helpful.

Fixes #<issue_number_goes_here> 🦕

Author: mbroz2

examples/helloworld/client/src/main/java/io/a2a/examples/helloworld/HelloWorldClient.java

@@ -7,9 +7,12 @@
 import io.a2a.client.A2AClient;
 import io.a2a.A2A;
 import io.a2a.spec.AgentCard;
+import io.a2a.spec.EventKind;
 import io.a2a.spec.Message;
 import io.a2a.spec.MessageSendParams;
 import io.a2a.spec.SendMessageResponse;
+import io.a2a.spec.Part;
+import io.a2a.spec.TextPart;
 
 /**
  * A simple example of using the A2A Java SDK to communicate with an A2A server.
@@ -50,7 +53,19 @@ public static void main(String[] args) {
                 .build();
             SendMessageResponse response = client.sendMessage(params);
             System.out.println("Message sent with ID: " + response.getId());
-            System.out.println("Response: " + response.toString());
+            
+            EventKind result = response.getResult();
+            if (result instanceof Message responseMessage) {
+                StringBuilder textBuilder = new StringBuilder();
+                if (responseMessage.getParts() != null) {
+                    for (Part<?> part : responseMessage.getParts()) {
+                        if (part instanceof TextPart textPart) {
+                            textBuilder.append(textPart.getText());
+                        }
+                    }
+                }
+                System.out.println("Response: " + textBuilder.toString());
+            }
         } catch (Exception e) {
             System.err.println("An error occurred: " + e.getMessage());
             e.printStackTrace();