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 ne nomme aucun fichier d’implémentation ni aucun test. Commencez par localiser les paramètres de l’application de bureau et la configuration de l’agent en arrière-plan, puis comparez le contrôle demandé avec le paramètre VSCode's GitHub › Copilot › Chat › Explore Agent: Model et examinez Issue #4178. C’est terminé lorsque les utilisateurs peuvent attribuer des modèles configurés ou BYOK à chaque agent en arrière-plan depuis un onglet de paramètres dédié.