Incorrect due dates on Canvas assignments

[noahnizamian]

General Info

original issue:

• Incorrect due dates on Canvas assignments berkeley-cdss/flextensions#275

Changes

Canvas API's due_at field can return incorrect values for teacher accounts when assignment overrides exist, so for a workaround we now fetch all_dates array and extract base date entry.
canvas for some reason imposes a limit of 25 on the number of dates returned by all_dates, and then returns an empty array.
so, this PR adds a fallback so that when base_date cant be extracted, the existing local due dates are preserved for already synced assignments instead of being overwritten with potentially incorrect top level due_at. New assignments still pick up the due dates from the API since theres no existing dates to preserve.

Testing

Added unit tests covering:

• base_date extraction from all_dates when present (uses correct base date)
• Empty all_dates array (>25 overrides): existing assignment dates are preserved
• Empty all_dates array for new assignments: dates are still set from the API
• Facade correctly handles both populated and empty all_dates arrays

Documentation

none needed

Checklist

• Name of branch corresponds to story