feat(ENT-12316): Add COURSE_RUN_ENDED assignment expiration reason

[gshivajibiradar]

Implementation summary:

• Added COURSE_RUN_ENDED to AssignmentAutomaticExpiredReason enum
• Implement _get_course_run_ended_date() to check if all known course runs have ended
• Implement catalog-agnostic content metadata lookup for archived/excluded courses
• Add automatic expiration logic that triggers when:
  • All known course runs for content have ended
  • Even with no enrollment deadline
  • Even when course excluded from active catalog
• Add backfill management command to expire existing assignments with ended runs
• Preserve existing expiration reasons and behavior

Test fixes:

• Fixed 3 datetime assertion tests by using future end_dates
• Test utils now correctly prioritize COURSE_RUN_ENDED over enrollment deadlines
• Updated mock metadata to prevent incorrect test failures