"Cancel coding agent" could use status

[connor4312]

Testing #7424

It took about 10 seconds for me for the "cancel coding agent" to complete. We should show a busy indicator somewhere for this, I wasn't entirely sure if it was still happening or if the button would just stay there disabled, until I eventually saw it disappear

[alexr00]

We should show a loading spinner in the button while it's disabled.

[alexr00]

To verify:

• Install the latest pre-release of GitHub Pull Requests
• Open your clone of a GitHub repo
• Prompt Copilot to create a PR by using #copilotCodingAgent in the Chat view and asking it to create a PR
• Open the PR description
• Use the "Cancel coding agent" button in the top right of the description to cancel.
• Verify that you see a progress spinner.

[mrleemurray]

Verified that it behaves as expected now - the spinner appears on click, then disappears once the agent has been cancelled.