Agents window: Can't create multiple sessions on same PR

Testing #332192

* Launch the Agents window
* Expand the "+" action on for the `vscode-internalbacklog` workspace and choose "New Session from Pull Request"
* Pick a pull request, and create a session for it. Use the draft pull request
* ✅ Session is created
* I tried the "+" action flow again to create another session on the same PR
* ❓ The list of PRs is empty
* Why can't I create more than one session on the same PR?
* If there are no available PRs, I'd like a message about that, rather than an empty quick pick