Fix Hide & Reveal settings dropped on non-deep-linked assignments

[Elimpizza]

This pull request updates how "Hide & Reveal" (checkpoint) assignment fields are handled and submitted in the LMS frontend. The main improvements are the consistent calculation and passing of the due date as a UTC ISO string, the addition of explicit hidden fields for checkpoint_enabled and due_date in the form, and enhanced test coverage for these behaviors.

Assignment workflow and form handling:

• The due date is now calculated once as a UTC ISO string (dueDateISO) and consistently passed through all submit paths and components, ensuring the backend always receives the correct format. [1] [2] [3] [4]
• The FilePickerFormFields component receives new props: checkpointEnabled (boolean) and dueDate (ISO string or null), making the assignment type and due date explicit in form handling. [1] [2]

Form field rendering:

• When checkpointEnabled is true, hidden fields for checkpoint_enabled and (if set) due_date are rendered in the form, ensuring these values are submitted only for the correct assignment type.

Testing improvements:

• New tests verify that the checkpoint fields are included or omitted as appropriate, and that the correct values are rendered for both checkpoint_enabled and due_date. [1] [2]