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.

L'issue non indica alcun file di implementazione né test. Inizia individuando le impostazioni dell'app desktop e la configurazione dell'agente in background, quindi confronta il controllo richiesto con l'impostazione VSCode's GitHub › Copilot › Chat › Explore Agent: Model ed esamina Issue #4178. Il lavoro è completato quando gli utenti possono assegnare modelli configurati o BYOK a ciascun agente in background da una scheda delle impostazioni dedicata.

Scritto dal modello di indicizzazione a partire dal testo della issue.