UI: Add Combobox.InputGroup primitive

[mirka]

What?

Adds the missing Combobox.InputGroup primitive to @wordpress/ui.

Why?

Base UI's Combobox supports an InputGroup wrapper for composing the input with prefix/suffix slots or other elements. Without it, the popup anchors to the bare <input> instead of the full control. The Autocomplete family already exposes this subcomponent.

Specifically, we'll need this component to solve the popover positioning problem raised in review for SearchableChipSelect.

How?

Add a thin InputGroup wrapper around Base UI's Combobox.InputGroup, matching the existing Autocomplete.InputGroup implementation.

Testing Instructions

1. Run Storybook for @wordpress/ui.
2. Open Design System / Components / Form / Primitives / Combobox.
3. Confirm Combobox.InputGroup appears in the subcomponents list.

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: simison <simison@git.wordpress.org>

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[github-actions]

Size Change: 0 B

Total Size: 7.76 MB

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[github-actions]

Flaky tests detected in 1115f4d.
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/30491091770
📝 Reported issues:

• [Flaky Test] should update the HTML only for the latest navigation #78835 in /test/e2e/specs/interactivity/router-navigate.spec.ts

[simison]

Looks clean and simple 👍