Course Outline: Add CSS class to lessons blocked by content dripping

[Drivingralle]

Resolves #7934

Proposed Changes

Add a pending CSS class to the lesson <a> element in render_lesson_block() when the lesson is blocked by content dripping for the current user. The check is guarded with function_exists( 'Sensei_Content_Drip' ) so it has no effect when the Content Drip module is inactive.

Testing Instructions

1. Set up a course with content dripping enabled on one or more lessons
   (absolute or dynamic drip type).
2. Log in as a student who is enrolled but does not yet have access to
   the dripped lesson.
3. View the course page and inspect the lesson link in the course
   outline — it should have both
   wp-block-sensei-lms-course-outline-lesson and pending in its class
   attribute.
4. Log in as a student who does have access — the pending class
   should not be present.
5. Deactivate the Content Drip module and confirm no errors occur and
   the class is absent.

New/Updated Hooks

• None

Deprecated Code

• None

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately
  describe the changes
• Adheres to coding standards ([PHP](https://developer.wordpress.or
  g/coding-standards/wordpress-coding-standards/php/),
  [JavaScript](https://developer.wordpress.org/coding-standards/wordpress
  -coding-standards/javascript/), [CSS](https://developer.wordpress.org/c
  oding-standards/wordpress-coding-standards/css/), [HTML](https://develo
  per.wordpress.org/coding-standards/wordpress-coding-standards/html/))
• All strings are translatable (without concatenation, handles
  plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first
  approach
• Code is tested on the minimum supported PHP and WordPress
  versions