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

No files or tests are named. Reproduce the flow by asking the app to open a pull request, closing it, and continuing the session; trace the pull-request update entry point and verify that closed PRs stop receiving edits or that a new PR is opened after confirmation.

索引モデルが issue の本文から書いたものです。