fix(generation): stabilize model picker teardown

[LeoParkerOu]

Summary

Make the generation toolbar model picker non-modal and close it before applying a model change, avoiding the focus-trap teardown race with nested controls.

Related Issues

Fixes #1447

Changes

• Configure the toolbar model-picker Popover with modal={false}.
• Close the model picker before updating the selected provider and model.
• Add browser regression coverage for click, Enter, and Space selection.
• Cover nested thinking controls, outside-click dismissal, Escape dismissal, and trigger focus restoration.
• Repeat the affected interactions to exercise the intermittent teardown path.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or build changes

Verification

Steps to reproduce / test

1. Open the generation toolbar model picker.
2. Select models using click, Enter, and Space, including after switching providers and opening the nested thinking selector.
3. Verify the picker closes after selection, outside-click dismissal works, Escape restores focus to the trigger, and no page error is raised.

What you personally verified

• Prettier, ESLint, and tsc --noEmit pass for the changed code.
• The dedicated Playwright suite passes: 5 tests, including ten-cycle stress coverage of the affected interactions.
• Click, keyboard selection, nested popup use, outside-click dismissal, and Escape focus restoration were tested locally.
• The original intermittent Windows 11/WSL2 exception was not reproduced locally; the regression suite exercises the reported focus and teardown paths.

Evidence

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally
• Screenshots / recordings attached (if UI changes)

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings

[wyuc]

Approving — thanks @LeoParkerOu, and for picking this up so quickly. Two-line production change with the right mechanism: no modal focus trap to tear down, and the picker closes before the provider/model update so the nested control isn't unmounting mid-selection. The e2e coverage is what makes modal={false} safe here — outside-click dismissal, Escape, and focus restoration to the trigger are exactly the behaviours that change when the trap goes away, and repeating the interaction exercises the intermittent path.