docs: fix extractTextFromMessage snippet to use record accessors (#713)

# Description

Replace getParts()/getText() with record accessor methods
parts()/text(), use typed Part<?> instead of raw Part, and remove
redundant null check that is already enforced by Message's compact
constructor.

Signed-off-by: Chaemin Lee <akak640@gmail.com>
Co-authored-by: Emmanuel Hugonnet <ehsavoie@users.noreply.github.com>

Author: CokeLee777

README.md

@@ -219,11 +219,9 @@ public class WeatherAgentExecutorProducer {
 
         private String extractTextFromMessage(Message message) {
             StringBuilder textBuilder = new StringBuilder();
-            if (message.getParts() != null) {
-                for (Part part : message.getParts()) {
-                    if (part instanceof TextPart textPart) {
-                        textBuilder.append(textPart.getText());
-                    }
+            for (Part<?> part : message.parts()) {
+                if (part instanceof TextPart textPart) {
+                    textBuilder.append(textPart.text());
                 }
             }
             return textBuilder.toString();