修复 Unsubscribe 方法中的回调比较逻辑，确保正确关闭队列并返回适当的错误信息

[taills]

This pull request improves the unsubscribe logic in the messageBus by making handler matching more precise and providing clearer error reporting when unsubscribing from a topic.

Error handling improvements:

• Updated the handler comparison in func (b *messageBus) Unsubscribe to use pointer equality (Pointer()), ensuring that the correct handler is identified and unsubscribed.
• Added specific error messages when a handler is not found for a topic or when the topic itself does not exist, making debugging easier.