Improve classic editor visibility controls layout

[poligilad-auto]

Trac ticket: https://core.trac.wordpress.org/ticket/65530

Summary

• Groups the classic editor Publish box Visibility radio options in a fieldset.
• Moves the conditional password field below the radio group while preserving the existing visibility save values.
• Keeps the Sticky checkbox as a conditional secondary control and adds focused layout coverage.

Context

This is intended as a small, incremental improvement to the classic editor Publish box that makes related Visibility controls easier to scan, moves the UI slightly closer to the block editor's explicit Visibility choices, and preserves the existing workflow and save behavior.

This PR does not change post status behavior or the meaning of public, password protected, or private visibility. The save path continues to use the existing visibility=public|password|private values.

Related tickets:

• Sync packages for WordPress 6.7 RC4 #7745 discusses the longer-standing Status/Visibility coupling for private posts. This PR is narrower and does not attempt to change private post status behavior.
• #25459 addressed accessibility improvements in the Publish box.
• #24792 touched the password-protected visibility password field.
• #47153 provides broader admin form control accessibility context.

Testing

• npm run test:php -- --filter Tests_Admin_IncludesMetaBoxes tests/phpunit/tests/admin/includesMetaBoxes.php
• php -l src/wp-admin/includes/meta-boxes.php
• php -l tests/phpunit/tests/admin/includesMetaBoxes.php
• node --check src/js/_enqueues/admin/post.js
• git diff --check

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[github-actions]

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