Copilot CLI does not prompt user to login if they are unauthenticated

[pierceboggan]

Sending message to Copilot CLI agent without signing in results in non-actionable error.

When you run the same to VS Code agent, we present the login dialog with GitHub, social auth, etc.

[pierceboggan]

FYI @cwebster-99 on growth

[cwebster-99]

Similar to #295901

[cwebster-99]

In local sessions, we trigger the sign in dialog and if dismissed show this text in chat. We should match this experience.

[cwebster-99]

Interesting I get a different response than you when I try with Copilot CLI

This is when the Copilot extension is installed but I am not signed in