feat(appserver): add turn event listener

Author: loong10k

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

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2018-present, easy-4-java (https://github.com/easy-4-java).
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ */
+package io.github.easy4j.codex.appserver;
+
+/**
+ * Listener for high-value Codex app-server turn events.
+ *
+ * <p>All methods are optional. Implementations should return quickly because
+ * callbacks are invoked on the WebSocket event path.</p>
+ */
+public interface CodexAppServerListener {
+
+    default void onTurnStarted(String turnId) {
+    }
+
+    default void onTextDelta(String delta) {
+    }
+
+    default void onItemCompleted(String itemType, String content) {
+    }
+
+    default void onTokenUsage(String rawJson) {
+    }
+
+    default void onWarning(String rawJson) {
+    }
+}