Desktop app: Add ability to pick BYOK/configured models for background agents

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

Background agents in GitHub Copilot Desktop currently run using an automatically selected model, and users cannot override or choose a different one.

### Proposed solution

Add model-selection control to the desktop app’s settings in a dedicated background agents tab in to assign models to each background agent. This would bring the desktop app to parity with VSCode, where the Explore Agent's model can be set by going to GitHub › Copilot › Chat › Explore Agent: Model

### Example prompts or workflows

_No response_

### Additional context

This request is related to Issue #4178, which focuses on showing the effective model used by background agents.

该 issue 没有指出任何实现文件或测试。首先定位桌面应用的设置和后台代理配置，然后将所请求的控制项与 VSCode's GitHub › Copilot › Chat › Explore Agent: Model 设置进行比较，并查看 Issue #4178。当用户可以从专用设置选项卡为每个后台代理分配已配置的模型或 BYOK 模型时，即视为完成。

由索引模型根据 Issue 内容生成。