agent: Show provider icon in model selectors

[dive]

I often switch between models, and I believe many people do. Currently, it is difficult to determine which provider offers the selected model because the same models are available from different providers. I propose a simple change to the selector so that users can distinguish between providers from the model they have chosen.

Before	After

Release Notes:

• The model selector has been improved with a provider icon, making it easier to distinguish between providers.

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[dive]

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