`copilot` should not use whatever valid token it can find if I set `GITHUB_TOKEN`

### Describe the feature or problem you'd like to solve

If I try to auth via env var, I expect `copilot` to use it even if it is invalid, and not to fallback to another source like the keyring or `gh`. This can result in highly confusing behaviour when scopes or accessible resources are different.

### Proposed solution

`copilot` should error, indicating that the token in was invalid.

### Example prompts or workflows

```
➜ runc git:(main) GITHUB_TOKEN=foo copilot -p "hello"
● Hello! I'm GitHub Copilot CLI, ready to help you with tasks in your runc repository. What would
you like to do?

Total usage est: 1 Premium request
Total duration (API): 3.4s
Total duration (wall): 5.2s
Total code changes: 0 lines added, 0 lines removed
Usage by model:
claude-sonnet-4.5 5.9k input, 33 output, 2.1k cache read, 3.8k cache write (Est. 1 Premium request)
```

### Additional context

_No response_