[FEATURE]: Fix V2 fast model exposure #34486

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Fast models should work correctly in V2 and should not be exposed as variants when they are actually separate model choices.

Context from the V2 beta discussion:

• The V2 TUI model switcher currently appears to treat fast models as variants.
• That is wrong for providers/models where fast mode is represented as a separate model.
• This makes model cycling/selection confusing and can prevent users from selecting the intended model.

Expected scope:

• Audit how V2 model/provider data represents fast models.
• Ensure fast models show up as separate models where appropriate.
• Ensure TUI/client model selection can cycle/select them correctly.
• Add tests for model list/selection shape.