Feature: Allow switching AI models within an existing chat session

[fengmoren-cyber]

Hi team, loving Wave Terminal so far!

I noticed that once an AI model is selected for a chat session,
it cannot be changed later. The only way to use a different model
is to create a brand new chat.

It would be great if we could switch the model dynamically within
an existing chat. This would be very helpful for:

1. Comparing responses from different models for the same prompt.
2. Using a cheaper/faster model for quick questions, and a smarter
   model for complex logic within the same context.

Thanks for considering this!