fix(ui): sync provider when selecting cross-provider model

[dpshde]

Summary

• sync the active provider when :model selects a model owned by a different provider
• prevent invalid session pairs like fireworks-ai + glm-5.1
• keep request routing consistent with the selected model instead of the previously active provider

Problem

Forge allowed cross-provider model selection from the all-provider model picker, but only persisted the new model. That made it possible to save impossible session state such as:

[session]
provider_id = "fireworks-ai"
model_id = "glm-5.1"

This caused confusing routing and misleading upstream errors because the selected model and active provider no longer matched.

Fix

When :model selects a model from another provider, resolve that model's owning provider and activate it before finalizing the model selection.

Verification

• reproduced the invalid provider/model session pair locally
• confirmed Forge routed incorrectly in the corrupted state
• confirmed valid Fireworks configuration routes to Fireworks after setting a valid provider/model pair
• confirmed the session config now stays provider/model consistent

Related

Closes #2859

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[amitksingh1490]

There were several places this issue could have been caused. With the latest changes, the model selection prompt now appears every time a user logs in. We also removed the separate provider selection, since both the model and provider can now be chosen directly through model selection.

Closing in favour of #2803