GitHub Copilot sessions should not continue to update closed PRs

### Short summary

GitHub Copilot sessions should stop updating PRs once they are closed.

### Affected version or release

v1.1.2

### Installation context

_No response_

### What happened?

I was asking GitHub Copilot to edit some files, it opened a PR at the end. However, I had to close the PR because of internal reasons.
Afterwards, the app still continued to make edits and updates to previously closed PRs, which seems very odd.

### Steps to reproduce

1 - Ask the app to open an PR
2 - Close it
3 - Continue to iterate
4 - You will notice the closed PRs are being updated still

### Expected behavior

It should ask users for confirmation about the PR, or just open a new one

### Additional context

This feedback was also referenced in some chinese social media posts

未指定文件或测试。通过请求 app 打开一个 pull request、将其关闭并继续会话来重现该流程；跟踪 pull-request 更新入口，并验证已关闭的 PRs 不再接收修改，或者在确认后会打开一个新的 PR。

由索引模型根据 Issue 内容生成。