Replace the custom field values for a ticket.

Important replace-all semantics: The request body must contain
all custom field values you want the ticket to have. Any field that
currently has a value on the ticket but is not included in the
request body will be deleted.

Some system-managed fields (e.g. AI Agent Outcome, AI Intent)
can only be deleted by Gorgias internal apps.

Safe pattern: fetch the current values with
GET /api/tickets/{ticket_id}/custom-fields, merge your changes, and
send the full list back.