Combobox, Select, SelectControl: Add Group and GroupLabel

[mirka]

What?

Adds Group and GroupLabel subcomponents to the Combobox and Select item-popup primitive families, and re-exports them from SelectControl.

Why?

Autocomplete already supports labeled item groups. Combobox and Select share the same item-popup styling but were missing equivalent subcomponents, so grouped options could not be composed consistently across these families.

How?

• Add group.tsx and group-label.tsx wrappers for Combobox and Select, following the existing Autocomplete implementation.
• Apply shared item-popup styles for group spacing and label typography.
• Re-export SelectControl.Group and SelectControl.GroupLabel.
• Add Storybook grouped examples and ref-forwarding unit tests.

Testing Instructions

2. Start Storybook and open:
   • Design System / Components / Form / Primitives / Combobox → Grouped
   • Design System / Components / Form / Primitives / Select → Grouped
   • Design System / Components / Form / SelectControl → Grouped
3. Confirm group labels render with heading-sm styling and items remain selectable.

Screenshots or screencast

[github-actions]

Size Change: +594 B (+0.01%)

Total Size: 7.75 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+594 B (+0.14%)

_{compressed-size-action}

[github-actions]

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

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[github-actions]

Flaky tests detected in 6ec5cb7.
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/29930298706
📝 Reported issues:

• [Flaky Test] should undo typing after non input change #78702 in /test/e2e/specs/editor/various/undo.spec.js

[aduth]

Confirmed that this functions as expected, follows pattern from Autocomplete Group & GroupLabel, and that the rationale for doing so is logical. 👍