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` 复现该问题，选择一个会话并运行 `/model`。跟踪恢复会话的模型列表请求及其身份验证处理；完成标准是 `/model` 能够加载模型列表，并允许在恢复的会话中切换模型，同时不影响其他命令。

由索引模型根据 Issue 内容生成。