CustomSelectControl V2 legacy adapter: fix trigger button font size

[ciampo]

What?

Part of #55023

Update the font size of the CustomSelectControl V2 legacy adapter to use the same underlying logic as V1

Why?

V2 legacy adapter should match V1 styles

How?

The V1 implementation re-uses some SelectControl styles, which in turn use a utility from InputControl to determine the font size.

I've updated CustomSelectControlV2 to use the same utility.

Testing Instructions

In Storybook, open CustomSelectControlV2 legacy adapter examples:

• Make sure that the font size is 16px regardless of the size prop for screens narrower than 600px
• Make sure that, for screens wider than 600px, the font size is 11px when the size is small, and 13px otherwise
• In general, make sure that the behavior is aligned with the V1 version of the component

Note

During my testing, I realised that CustomSelectControl V1 has a bug for which the font size doesn't change as it should according to the size for screens wider than 600px — for some reason, the size doesn't get correctly passed to the fontSizeStyles utility, and as a consequence the default font size is always applied (13px). You can verify the buggy behaviour by setting size="small" on CustomSelectControl V1 and observe how the font size is still 13px instead of 11px.
I don't think we should spend time fixing it, since the V1 implementation is soon going away.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[tyxla]

Confirmed to always be 16px on mobile layouts, just like on the v1:

Small size:

Default size:

(Unstable) Large size:

Font size is 16px for all: ✅