feat: Return List instead of ImmutableList in CallbackUtil methods

PiperOrigin-RevId: 886725038

Author: google-genai-bot

core/src/main/java/com/google/adk/agents/BaseAgent.java

@@ -529,7 +529,8 @@ public B beforeAgentCallback(BeforeAgentCallback beforeAgentCallback) {
 
     @CanIgnoreReturnValue
     public B beforeAgentCallback(List<Callbacks.BeforeAgentCallbackBase> beforeAgentCallback) {
-      this.beforeAgentCallback = CallbackUtil.getBeforeAgentCallbacks(beforeAgentCallback);
+      this.beforeAgentCallback =
+          ImmutableList.copyOf(CallbackUtil.getBeforeAgentCallbacks(beforeAgentCallback));
       return self();
     }
 
@@ -541,7 +542,8 @@ public B afterAgentCallback(AfterAgentCallback afterAgentCallback) {
 
     @CanIgnoreReturnValue
     public B afterAgentCallback(List<Callbacks.AfterAgentCallbackBase> afterAgentCallback) {
-      this.afterAgentCallback = CallbackUtil.getAfterAgentCallbacks(afterAgentCallback);
+      this.afterAgentCallback =
+          ImmutableList.copyOf(CallbackUtil.getAfterAgentCallbacks(afterAgentCallback));
       return self();
     }
 

core/src/main/java/com/google/adk/agents/CallbackUtil.java

@@ -42,7 +42,7 @@ public final class CallbackUtil {
    * @return normalized async callbacks, or empty list if input is null.
    */
   @CanIgnoreReturnValue
-  public static ImmutableList<BeforeAgentCallback> getBeforeAgentCallbacks(
+  public static List<BeforeAgentCallback> getBeforeAgentCallbacks(
       List<BeforeAgentCallbackBase> beforeAgentCallbacks) {
     return getCallbacks(
         beforeAgentCallbacks,
@@ -59,7 +59,7 @@ public static ImmutableList<BeforeAgentCallback> getBeforeAgentCallbacks(
    * @return normalized async callbacks, or empty list if input is null.
    */
   @CanIgnoreReturnValue
-  public static ImmutableList<AfterAgentCallback> getAfterAgentCallbacks(
+  public static List<AfterAgentCallback> getAfterAgentCallbacks(
       List<AfterAgentCallbackBase> afterAgentCallback) {
     return getCallbacks(
         afterAgentCallback,