resumed session blocks access to model picker

### Describe the bug

When I resume a session via `copilot --resume`, if I try to use `/model` command in the resumed session, all attempts are blocked with

```
✗ Error loading model list: Error: Not authenticated
```

All other commands in copilot work fine, and I can continue talking to previously selected model as well, but I'm not able to pick a new model in the resumed session due to this.

### Affected version

1.0.59

### Steps to reproduce the behavior

1. `copilot --resume`
2. pick session
3. resume session
4. try to switch models with `/model`
5. command is blocked with `✗ Error loading model list: Error: Not authenticated`

### Expected behavior

I can use `/model` in a resumed session.

### Additional context

_No response_

Reproduce the issue with `copilot --resume`, select a session, and run `/model`. Trace the resumed-session model-list request and its authentication handling; done means `/model` loads the model list and allows switching models in a resumed session without affecting other commands.

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