docs: Fix the error in implementing custom callback handler method in qiuckstart modification

[Claran309]

Summary

This PR fixes a technical error in the Quickstart documentation regarding the implementation of custom callback handlers.

Related Issues
Fixes #188

Changes

File	Change
quickstart\chatwitheino\docs\ch06_callback.md	Replaced the non-existent NewHandlerHelper with the correct NewHandlerBuilder method from the callbacks package.

Key Insight

In the core github.com/cloudwego/eino/callbacks package, the method for creating generic handlers is NewHandlerBuilder(), not NewHandlerHelper().

The original documentation provided a code snippet that would lead to a compilation error because NewHandlerHelper is either missing from that scope or belongs to a different utility package with a different calling convention. This update ensures that developers following the Quickstart guide can successfully implement their first callback handler without encountering "method not found" errors.

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概述

本 PR 修复了快速入门（Quickstart）文档中关于自定义回调处理器（Callback Handler）实现的一个技术错误。

关联议题
修复 #188

变更内容

文件	变更
quickstart\chatwitheino\docs\ch06_callback.md	将不存在的 NewHandlerHelper 替换为 callbacks 包中正确的 NewHandlerBuilder 方法。

核心洞察

在 Eino 的核心包 github.com/cloudwego/eino/callbacks 中，用于创建通用处理器的正确方法是 NewHandlerBuilder()。

原文档引用的 NewHandlerHelper 会导致开发者在按照教程操作时遇到编译错误（方法未定义）。通过此次修改，纠正了 API 的调用名称，确保新用户在学习快速入门指南时能够顺利运行示例代码，提升了文档的准确性和易用性。

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