Partially update an existing program cancellation. All fields are optional.

Business Rules:

• The program must not be published. Updates are only allowed for draft or ready-to-publish programs.
• If end is provided, start must also be provided in the same request.
• If start is provided without end, any previously set end date is removed (cancellation becomes single-day).
• The same date range validations as Create Cancellation apply when start is provided.