Ability to delete last few messages / go to a checkpoint

### Feature summary

Ability to delete last few messages / go to a checkpoint

### What problem are you trying to solve?

In VSCode, you can go to a certain checkpoint and that will effectively delete later messages and restore your conversation from that point. It is a very useful feature as sometimes the conversation takes a detour and we want to go back without needing to start a new one.

### Proposed solution

Follow what's been done in VSCode. Understandably, here we don't want to revert code, just move back to a certain point in conversation.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

该 issue 未指出文件、测试或入口点。首先将应用的对话流程与 VSCode 的检查点行为进行比较，然后定义选择先前某个时间点后对话会如何变化；完成标准是删除后续消息，并从该时间点继续对话。

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