Use a new quick chat for each request

[aeschli]

Testing #6321

• select an issue and invoke 'Fix with Copilot'. Quick chat opens and proposes something
• select a different issue and invoke 'Fix with Copilot'. Quick chat adds that suggestion under the previous conversation.

As the two entries are not related to each other, I would prefer a new chat. With each request, the quick chat gets larger and difficult to navigate

[alexr00]

To verify:

• Install the latest pre-release version of GitHub Pull Requests
• Open a folder which contains a GitHub repo
• Open the Chat view and start a chat
• Open the "Issues" view
• Right click on one of the issues and "Suggest a fix with Copiliot"
• Verify that a new Chat is used