Copilot Chat reports an invalid GitHub token while VS Code still shows the account as signed in

### Problem

VS Code shows this Copilot notification:

> Your GitHub token is invalid. Please sign out from your GitHub account...

However, the Accounts/Profile UI shows the GitHub account as **signed in** and does not show a **Sign in** action or another clear way to refresh the token used by Copilot Chat.

### Actual behavior

- Copilot Chat enters a non-working state after the invalid-token notification appears.
- The UI indicates that every relevant account is already signed in, so there is no apparent recovery path.
- Signing out and signing back in can eventually resolve the problem, but it often requires several window reloads and retries before Copilot starts working again.
- There is no reliable indication of when Copilot has returned to a usable authenticated state.

### Expected behavior

When Copilot detects an invalid GitHub token, VS Code should provide a clear, actionable recovery path, such as:

- a **Sign in again** / **Refresh GitHub token** action directly in the notification;
- an Accounts UI state that accurately reflects the invalid Copilot authentication; and
- clear confirmation when Copilot authentication is restored.

### Impact

This makes Copilot Chat unnecessarily difficult to use: recovery becomes trial-and-error (sign out, sign in, reload, retry) rather than a deterministic re-authentication flow.

I can provide VS Code, Copilot, and GitHub authentication extension versions if needed.