Investigate and fix incorrect Canvas assignment due dates, add logging for overrides

[cycomachead]

General Info

• Pivotal Tracker Story
• Breaking?

Changes

Canvas omits the all_dates array (the source of the base due date) from its API response when an assignment has more than ~25 overrides. Without a base date, the sync was falling back to the root-level due_at/lock_at fields, which Canvas can populate from an override — resulting in due dates that are later than the real assignment due date.

Root cause fix: Skip overwriting due_date and late_due_date on subsequent syncs when an assignment has ≥ 25 overrides and no explicit base date is present. New assignments are unaffected (dates are always set on first sync), and non-Canvas LMS integrations (e.g. Gradescope) are unaffected via safe defaults in BaseAssignment.

Key changes:

• SyncAllCourseAssignmentsJob: Added OVERRIDE_DATE_LIMIT = 25 constant and skip_due_date_update? guard. Name and other fields continue to sync normally even when date updates are skipped.
• Lmss::Canvas::Assignment: Exposes overrides_count (size of the overrides array) and base_date? (whether Canvas returned an explicit base date).
• Lmss::BaseAssignment: Added default implementations (overrides_count = 0, base_date? = true) so other LMS integrations are never affected by the guard.
• Logging: A warn is emitted when a due date update is skipped (showing stored vs. Canvas-reported dates). An info is emitted for any override-bearing assignment whose dates are updated, so we can continue monitoring for incorrect dates.
• CanvasSpecHelpers: Fixed build_canvas_assignment so passing base_date: nil correctly models Canvas omitting the base date, rather than deriving one.

Testing

Added RSpec contexts to sync_all_course_assignments_job_spec.rb covering:

• Many overrides + no base_date: Existing assignment's due dates are preserved on re-sync; name still updates. New assignment still gets dates set. Skip is logged with a warn.
• Many overrides + base_date present: Due dates are correctly updated from the base date even when override count is high.

All new and existing specs should be run against a database-backed environment before merging.

Documentation

No additional documentation required.

Checklist

• Name of branch corresponds to story

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