Editor: Improve Classic Editor status dropdown layout

[poligilad-auto]

Summary

• Keeps the Classic Editor publish box Status control as the legacy select#post_status to preserve existing extensibility contracts.
• Makes the Status dropdown full width in the publish box and moves OK/Cancel below it for spacing consistency.
• Adds focused regression coverage to protect the select#post_status markup contract.

This is scoped to the Status control layout only. Visibility UI changes are intentionally not included here.

Trac ticket: https://core.trac.wordpress.org/ticket/65532

Testing

• LOCAL_PORT=18089 npm run test:php -- --filter Tests_Admin_IncludesMetaBoxes
• vendor/bin/phpcs --standard=phpcs.xml.dist src/wp-admin/includes/meta-boxes.php tests/phpunit/tests/admin/includesMetaBoxes.php
• npx grunt jshint:core --file=js/_enqueues/admin/post.js
• npx grunt postcss:core
• npm run build:dev
• git diff --check
• curl -I --max-time 10 http://localhost:18089
• Playwright verification on http://localhost:18089/wp-admin/post-new.php confirmed Classic Editor, select#post_status, no fieldset#post_status, full-width dropdown, and OK/Cancel below the dropdown.

[github-actions]

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[github-actions]

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[poligilad-auto]

I changed the direction of this PR after doing an extensibility audit of the Classic Editor Status control.

The original version replaced the legacy select#post_status with a radio group. That preserves the submitted post_status values for Core’s own statuses, but it does not preserve the practical DOM contract that Classic Editor integrations rely on.

The audit found real plugin code that depends on select#post_status as an implicit extension point:

• GeoDirectory appends custom status <option> elements and reads the selected option to update the Save button text.
• AnsPress appends moderate and private_post options to select#post_status.
• WPAdverts appends/selects an expired option and changes submit behavior based on the selected option.
• BEA Post Status Admin appends custom statuses to the dropdown.
• PublishPress Statuses reads and manipulates #post_status as a select throughout its Classic Editor integration.
• WordCamp CampTix Invoices appends refunded and cancelled options to select#post_status.

Core also still has assumptions around #post_status behaving like a select, for example in local autosave status handling.

Given that, this PR no longer changes the control type. It now keeps the existing dropdown and limits the change to safer layout improvements:

• keep select name="post_status" id="post_status" intact;
• make the dropdown full width in the publish box;
• move OK/Cancel below the dropdown for spacing consistency;
• add a regression test that protects the legacy select markup contract.

That makes this a smaller visual improvement while avoiding the compatibility break from removing select#post_status.