Course Outline: Add CSS class to lessons blocked by an incomplete prerequisite

[Drivingralle]

Resolves #7936

Proposed Changes

Add a blocked CSS class to the lesson <a> element in render_lesson_block() when Sensei_Lesson::is_prerequisite_complete() returns false for the current user. Follows the same pattern as the existing completed class and the pending class proposed in #7934.

Testing Instructions

1. Set up a course with at least two lessons where the second lesson has the first set as a prerequisite.
2. Log in as a student who is enrolled but has not yet completed the first lesson.
3. View the course page and inspect the second lesson's link in the course outline — it should have both wp-block-sensei-lms-course-outline-lesson and blocked in its class attribute.
4. Complete the first lesson, then revisit the course outline — the blocked class should no longer be present on the second lesson.
5. Verify that lessons without a prerequisite are unaffected.

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, JavaScript, CSS, 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

[Copilot]

Pull request overview

Adds a new CSS hook to the Course Outline lesson link markup so themes can visually indicate lessons that are locked due to an incomplete prerequisite.

Changes:

• Add a blocked CSS class to lesson <a> elements when Sensei_Lesson::is_prerequisite_complete() is false for the current user.

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