DropdownMenuV2Ariakit: prevent prefix collapsing if all radios or checkboxes are unselected

[ciampo]

What?

Improves spacing in radio and checkbox menu items, so that the prefix space doesn't collapse if all items are unchecked

Why?

Currently, the whole menu can "jump" when selecting an item (as reported in #55625 (comment)), which is a poor user experience.

How?

By making sure that the prefix has a minimum width for radio and checkbox menu items

Testing Instructions

• Open the "With Checkboxes" Storybook example for the DropdownMenuV2Ariakit component
• Uncheck all checkboxes
• Make sure that all checkboxes are still showing some empty space as their prefix, and that the menu's width doesn't change
• To test with radios, manually tweak the "With Radios" storybook example to remove any initial selection

Screenshots or screencast

Before (trunk)	After (this PR)

[oandregal]

This works nicely, thanks.

[github-actions]

Flaky tests detected in 33335e2.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7142946584
📝 Reported issues:

• [Flaky Test] should display the "Manage Fonts" icon #56776 in /test/e2e/specs/site-editor/font-library.spec.js