TICKET-583: Include due date in LTI gradebook sync

[Naragod]

Proposed Changes 🐛 Bug fix

When creating or updating a gradebook item via LTI, MarkUs sends a line item payload to Canvas containing the assignment label, resource ID, and maximum score. The dueAt field supported by the LTI Assignment and Grade Services (AGS) specification was missing from this payload, and the due date set in MarkUs remained absent on the corresponding Canvas assignment. The fix adds dueAt as an ISO 8601 timestamp to the payload when the assessment has a due date, ensuring it is synced during both line item creation and updates.

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[coveralls]

Pull Request Test Coverage Report for Build 23362414972

Details

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 91.676%

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Totals
Change from base Build 23091690132:	0.001%
Covered Lines:	45084
Relevant Lines:	48364

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💛 - Coveralls