IMPORTANT: This is a full-replace operation (PUT semantics). All fields must be
included in the request body — omitting any field will set it to null, wiping
existing data. Always call retrieve-a-bill first to fetch the current record,
then include ALL existing fields in your update request along with your changes.
Bills with line allocations return 422 BillHasSubsidiaryAllocations, even for a no-op update.
These allocations cannot be sent through this API; edit the bill in Rillet instead.