feat(appserver): expose listener on turn request

Author: loong10k

src/main/java/io/github/easy4j/codex/appserver/AppServerTurnRequest.java

@@ -59,6 +59,12 @@ public class AppServerTurnRequest {
      */
     private Consumer<String> onTurnStarted;
 
+    /**
+     * Optional structured listener for turn lifecycle and streaming events.
+     * Existing callbacks remain supported for source compatibility.
+     */
+    private CodexAppServerListener listener;
+
     /**
      * Returns the session key in a comparable form, or {@code null} when unset.
      *