Course completion and certificates

[scottgrayson]

Details

Fixes three bugs:

1. Course completion impossible when no test steps exist: maybeSetCompletedAtForUser() now skips the required_test_percentage check if no test steps are present, allowing completion. The CourseCompleted page also applies this logic.
2. Unused property urlToFirstStepBelowPerfect: Removed the unused $urlToFirstStepBelowPerfect property and its assignment from CourseCompleted to eliminate dead code.
3. Certificate download allowed after failing retake tests: retakeTest() now clears the course's completed_at timestamp on the user pivot table, ensuring re-evaluation of completion status and preventing certificate download if the user no longer qualifies.

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Note

Medium Risk
Touches core course completion/certificate eligibility logic and modifies pivot completed_at state on retake, which could affect user progress if edge cases are missed; scope is small and localized.

Overview
Fixes course completion/certificate eligibility edge cases around test requirements and retakes.

Course::maybeSetCompletedAtForUser() and the CourseCompleted page now skip the required_test_percentage gate when a course has no test steps, preventing users from being blocked from completing a course/certificate due to a missing test set. TestStep::retakeTest() now clears the user’s course pivot completed_at so completion (and certificate availability) is re-evaluated after a retake, and dead code on CourseCompleted ($urlToFirstStepBelowPerfect) is removed.

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