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_

使用 `copilot --resume` 重現此問題，選取一個工作階段並執行 `/model`。追蹤已恢復工作階段的模型清單請求及其驗證處理；完成條件是 `/model` 能載入模型清單，並允許在已恢復的工作階段中切換模型，同時不影響其他命令。

由索引模型根據 Issue 內容生成。