fix(app): restore v2 reasoning selector

[Ekul34]

Issue for this PR

Fixes #29051.

This is the same missing V2 reasoning/model variant selector issue attempted in #29050, but that PR now conflicts with current dev after the prompt input was refactored. This applies the fix to the current Composer*Control structure.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Restores the model variant/thinking-effort selector in the new V2 prompt input when the selected model exposes multiple variants. It reuses the existing variant list, default-label behavior, selection update logic, and model.variant.cycle keybind tooltip.

How did you verify your code works?

• Ran bun typecheck from packages/app after applying a local-only Windows symlink checkout workaround for src/custom-elements.d.ts.
• Full pre-push typecheck was attempted, but current dev has unrelated packages/opencode event-type errors outside this UI change.
• git diff --check passed before commit.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PR Found

PR #29050: fix(app): show reasoning selector in v2 prompt
#29050

Why it's related: This is the predecessor PR that attempted the same fix for the missing V2 reasoning/model variant selector (issue #29051). PR #30063 is a reimplementation of this fix adapted to the current codebase after the prompt input was refactored. The current PR specifically addresses the fact that #29050 now conflicts with dev due to the Composer*Control structure changes.

[github-actions]

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