Add Agentic chat toggle to model selector dropdown

What does this MR do and why?

You can now switch between Agentic and Non-agentic Chat right from the model selector, next to the send button. Before, the switch sat in a separate spot and only went one way. Now it lives where you already look to change models, so moving between the two modes takes one click in a predictable place.

In Non-agentic Chat the selector opens a short dropdown that says model selection does not apply and offers a toggle back to Agentic Chat. That keeps the control consistent across both modes and avoids showing a model list that has no effect.

All of this stays behind the new_model_selection feature flag.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag in the Rails console:

   Feature.enable(:new_model_selection)

2. Open GitLab Duo Chat.
3. In Agentic Chat, open the model selector next to the send button and use the Agentic chat toggle to switch to Non-agentic Chat.
4. In Non-agentic Chat, open the selector, confirm the model-selection message, and toggle back to Agentic Chat.

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