CustomSelectControl: Restore describedBy functionality

[mirka]

Fixes #63568

What?

Restores the describedBy functionality in CustomSelectControl (v1) that was broken when we switched the underlying implementation in #63258.

How?

See code comments.

Testing Instructions

New unit tests should pass. You can also manually inspect the rendered HTML in Storybook.

[mirka]

The original implementation passed the describedBy prop down to Button. Therefore, we want to replicate how Button rendered the describedBy text.

gutenberg/packages/components/src/button/index.tsx

Lines 282 to 284 in cad81e4

	<VisuallyHidden>
	<span id={ descriptionId }>{ description }</span>
	</VisuallyHidden>

[mirka]

This function copies the original logic, which looked like this:

gutenberg/packages/components/src/custom-select-control/index.js

Lines 130 to 141 in 9bb75b3

	function getDescribedBy() {
	if ( describedBy ) {
	return describedBy;
	}
	if ( ! selectedItem ) {
	return __( 'No selection' );
	}
	// translators: %s: The selected option.
	return sprintf( __( 'Currently selected: %s' ), selectedItem.name );
	}

I removed the case for no selection, because that won't happen in this Ariakit-based version.

[mirka]

The Ariakit store allows string | string[] because it can handle multiple selection. Since this v1 never supported multiple selection, let's narrow the type to just string. (This has the effect of narrowing the type of store.getState().value to string.)

[github-actions]

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

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[tyxla]

✅ Code looks good, and this tests well:

Without describedBy	With describedBy

[afercia]

Please remember to manually add props to @rohitmathur-7 for the alternative PR at #63884