[Bug]: the `maxSubtasks` param of PlanNoteBook doesn't work

[tcsp]

Describe the bug
I has set the maxSubtasks param of PlanNoteBook, but it doesn't work.

To Reproduce

I set maxSubtasks to 5.

PlanNotebook planNotebook = PlanNotebook.builder()
                // 最大的子任务数量
                .maxSubtasks(5)
                // 是否需要用户确认执行
                .needUserConfirm(true)
                .build();

But when the reasoning request generates plan, the number of subtasks still exceed 5.

Environment (please complete the following information):

• AgentScope-Java Version: 1.0.7,
• Java Version: 17