Categories: Apply the legacy Categories widget filters - #85472

What?

Closes #40495.

The Categories block now applies the widget_categories_args and widget_categories_dropdown_args filters from the legacy Categories widget when it lists categories, so filters written for the widget also affect the block.

Why?

Sites that moved from the Categories widget to the Categories block lost the ability to change the block's output with these filters, for example to exclude categories or show empty ones. There was no other way to change the arguments the block passes to wp_list_categories() and wp_dropdown_categories().

Other blocks that replaced a legacy widget already reuse its filters: the Archives block applies widget_archives_args and widget_archives_dropdown_args, and the Latest Comments block applies widget_comments_args. The Categories block was the exception.

Testing Instructions

1. Add a mu-plugin (for example, wp-content/mu-plugins/categories-filters.php) with:

   <?php
   function test_categories_filters( $args ) {
   	$args['exclude']    = array( 1 );
   	$args['show_count'] = true;
   	return $args;
   }
   add_filter( 'widget_categories_args', 'test_categories_filters' );
   add_filter( 'widget_categories_dropdown_args', 'test_categories_filters' );

2. Make sure at least one category besides Uncategorized has a published post, and that a tag is assigned to a published post.
3. Add a Categories block to a page and view the page on the front end. Uncategorized (ID 1) is no longer listed, and the other categories show post counts. On trunk, Uncategorized is listed and there are no counts.
4. Turn on "Display as dropdown" and view the page again. The dropdown also leaves out Uncategorized and shows post counts.
5. Change the block's taxonomy to Tags, both as a list and as a dropdown. Tags are listed without counts, because the widget filters are not applied to other taxonomies.

Use of AI Tools

This PR was authored with the help of Claude Code.