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.

El issue no menciona archivos de implementación ni pruebas. Empieza por localizar la configuración de la aplicación de escritorio y del agente en segundo plano, luego compara el control solicitado con la configuración VSCode's GitHub › Copilot › Chat › Explore Agent: Model y revisa Issue #4178. Se considerará terminado cuando los usuarios puedan asignar modelos configurados o BYOK a cada agente en segundo plano desde una pestaña de configuración dedicada.

Escrito por el modelo de indexación a partir del texto del issue.