feat(hamplard): add course waitlist management with automatic promotion

[Folex1275]

Fix Test Coverage and Implement Waitlist Feature

Summary

This PR addresses three issues related to test coverage and missing functionality in the Hamplard contract:

1. Issue test.rs does not test archive_course() when course has pending student completions #142: Added test for archive_course() with pending student completions
2. Issue test.rs does not test behavior when DataKey::DefaultFee is missing at enroll time #143: Added test for missing DefaultFee at enrollment time
3. Issue lib.rs does not implement course waitlist with automatic enrollment on capacity opening #147: Implemented course waitlist with automatic enrollment promotion

Changes

Issue #142: Test archive_course() with Pending Completions

Problem: The test suite did not cover the scenario where archive_course() is called on a course with enrolled students who have not been marked as completed. These students are silently left in an uncompletable state after archival.

Solution:

• Added test_archive_with_pending_student_completions() - verifies that archiving a course with a mix of completed and pending students correctly refunds only the pending students while preserving completed enrollments
• Added test_mark_completed_on_archived_course_fails() - verifies that attempting to mark a student as completed after course archival produces a clear error

Files Modified:

• contracts/hamplard/src/test.rs

Issue #143: Test Missing DefaultFee Behavior

Problem: The test suite did not test the scenario where enroll() is called before DataKey::DefaultFee has been initialized. This could result in confusing panics in production if init is skipped.

Solution:

• Added test_enroll_missing_default_fee_produces_clear_error() - verifies that attempting to register a course with a custom fee below the platform default (when DefaultFee is missing) produces a clear, actionable error message

Files Modified:

• contracts/hamplard/src/test.rs

Issue #147: Implement Course Waitlist

Problem: When a course reaches its max_capacity, students who arrive later have no recourse. There was no waitlist mechanism to queue interested students and automatically enroll them when a spot opens.

Solution:

• Added DataKey::CourseWaitlist(String) to storage keys for per-course waitlist storage
• Implemented join_waitlist() - allows students to join the waitlist for a full course
• Implemented leave_waitlist() - allows students to leave the waitlist
• Implemented get_waitlist() - returns the ordered list of waitlisted students
• Implemented promote_from_waitlist() (internal) - automatically promotes the first waitlisted student when capacity opens
• Integrated waitlist promotion into archive_course() when refunds create available capacity
• Added comprehensive test coverage:
  • test_join_waitlist_success()
  • test_join_waitlist_rejects_when_capacity_available()
  • test_join_waitlist_duplicate_rejected()
  • test_leave_waitlist_success()
  • test_leave_waitlist_not_on_list()
  • test_waitlist_promotion_on_refund()

Files Modified:

• contracts/hamplard/src/lib.rs
• contracts/hamplard/src/test.rs

Testing

All new tests pass and verify:

• Archive behavior with pending completions is well-defined
• Missing DefaultFee scenario produces clear error messages
• Waitlist join/leave operations work correctly
• Automatic promotion from waitlist occurs when capacity opens
• Edge cases (duplicates, already enrolled, etc.) are properly handled

Closes

Closes #142
Closes #143
Closes #147

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