Fix: Respect hidden attribute in Shortcode block render - #78720

What?

Closes #78701

The Shortcode block render callback does not check the hidden
attribute, causing previously hidden shortcode blocks to remain
visible on the front end even in incognito mode.

Why?

PR #76138 disabled visibility support for the Shortcode block.
However, users who had previously hidden shortcode blocks still
have {"hidden":true} saved in their database from before that
change. The render callback was not checking this attribute,
causing those blocks to render on the front end even when
marked as hidden.

How?

Added a check for the hidden attribute in the render callback
in index.php. If the block is hidden, the function returns an
empty string instead of rendering the shortcode content.

function render_block_core_shortcode( $attributes, $content ) {
    if ( ! empty( $attributes['hidden'] ) ) {
        return '';
    }
    return wpautop( $content );
}

Testing Instructions

1. Open a post or page
2. Add a Shortcode block
3. Switch to Code Editor (3 dots → Code Editor)
4. Manually add {"hidden":true} to block markup:
   <!-- wp:shortcode {"hidden":true} -->[your_shortcode]<!-- /wp:shortcode -->
5. Publish the page
6. Visit the page in incognito mode

Before fix: Shortcode renders visibly on front end ✗

After fix: Nothing renders — block correctly hidden