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.

The issue names no implementation files or tests. Start by locating the desktop app's settings and background-agent configuration, then compare the requested control with VSCode's GitHub › Copilot › Chat › Explore Agent: Model setting and review Issue #4178. Done means users can assign configured or BYOK models to each background agent from a dedicated settings tab.

Written by the indexing model from the issue text.