story: Align combobox custom trigger carets (#2607)

## Summary

- expose and re-export the shared `Caret` component for custom Combobox
triggers
- use `Caret::new(ctx.size)` in the Item with Icon and Custom Trigger
examples
- keep custom trigger caret sizing consistent with the Combobox size
mapping

## Test Plan

- `cargo check -p gpui-component-story`
- `git diff --check`

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Co-authored-by: Codex <codex@openai.com>