[AI Bundle] Autowiring of AgentInterface broken for single agent config

After #281 it looks like it is no longer possible to use `AgentInterface $agent` anymore in case that only one agent is configured.

<img width="1100" height="363" alt="Image" src="https://github.com/user-attachments/assets/e17914af-adc9-4cd3-89dc-223de47acc84" />

This is a rather inconvenient regression.