Fix: Navigation Overlay Close announces the author's own text - #82227

What?

With Display Mode set to icon, the Navigation Overlay Close button is always
announced as "Close", whatever the author wrote in its text field.

$text = empty( $attributes['text'] ) ? __( 'Close' ) : $attributes['text'];
…
! $show_text ? 'aria-label="' . __( 'Close' ) . '"' : '',

The authored text is computed on the first line and the label ignores it.

Why?

icon is the default display mode, and it is the one mode where the label is the
button's entire accessible name — there is no visible text to fall back on. So an
author who writes "Dismiss menu" sees it in the text and both modes and loses
it in the mode where it matters most.

This looks like something left behind rather than a decision. When the block was
added, text was a rich-text attribute with "default": "Close", which made the
fixed label and the authored one the same string. text is now a plain string with
no default, and the label stopped following it.

The Search block already resolves the same situation the other way, naming its
icon-only button from the text the author gave it:

$button->set_attribute( 'aria-label', wp_strip_all_tags( $attributes['buttonText'] ) );

How?

The label uses the text that was already computed, with markup stripped, since an
attribute is not a place tags can live. The default is untouched: an author who has
written nothing still gets "Close".

The editor preview carried the same fixed string with the value it should use
already in scope, so it changes with it and the two stay in agreement.

Testing Instructions

1. Add a Navigation block with an overlay, and select the Navigation Overlay Close
   block inside it.
2. Set Display Mode to Text, and change the text to "Dismiss menu".
3. Set Display Mode back to Icon.
4. Inspect the button on the front end: aria-label="Dismiss menu".
5. Clear the text and confirm the label falls back to "Close".
6. In Text and Both modes there is no aria-label, because the visible text is
   already the button's name and a label would repeat it.

phpunit/blocks/render-block-navigation-overlay-close-test.php covers the four
cases: authored text in icon mode, the empty fallback, markup stripped from the
label, and no label when the text is visible.

OK (4 tests, 9 assertions)

Both changed files pass phpcs --standard=phpcs.xml.dist.

Screenshots or screencast

Rendered with displayMode: "icon" and text: "Dismiss menu":

<!-- before -->
<button type="button" aria-label="Close"><svg …/></button>

<!-- after -->
<button type="button" aria-label="Dismiss menu"><svg …/></button>

A note on scope

This is the accessibility half of one small thing, and I have deliberately not
touched the rest of it here.

While looking into it I noticed that Core spells "show an icon, text, or both" four
different ways across blocks that all render an action control:
displayMode: icon|text|both here, buttonUseIcon in Search, hasIcon + icon in
Navigation, and showIcon + iconPosition in Accordion — with the lightbox
controls having no attribute at all. Their supports differ too: this block has
color, spacing and typography but no border, dimensions or shadow, so a
theme cannot give an icon-only control a shape or a container size.

That is a larger conversation, and it overlaps with
#79057 on Button icons and
with the Icon Registry work in
#81225 /
#82062. I would rather raise it
separately than widen this fix, which stands on its own.