fix: correct callback API in ch06 documentation

[123456wda]

Summary

• Fix incorrect callback API in ch06_callback.md documentation
• The code example used callbacks.NewHandlerHelper() with .OnStart()/.OnEnd()/.OnError()/.Handler(), which does not match the actual API
• Corrected to callbacks.NewHandlerBuilder() with .OnStartFn()/.OnEndFn()/.OnErrorFn()/.Build()
• Bump eino and eino-ext dependency versions

Background

The quickstart chapter 6 callback example code was inconsistent with the actual github.com/cloudwego/eino/callbacks package API. The correct API for building a generic callback handler is:

handler := callbacks.NewHandlerBuilder().
    OnStartFn(...).
    OnEndFn(...).
    OnErrorFn(...).
    Build()

Note: NewHandlerHelper() lives in github.com/cloudwego/eino/utils/callbacks and is for component-specific typed callbacks (e.g., .ChatModel(), .Tool()), not for generic callbacks.

Test plan

• Review the corrected code examples in ch06_callback.md
• Verify the API matches github.com/cloudwego/eino/callbacks v0.8.13

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