Update loan fields. Every changed field is recorded in the loan's event
history with its before and after value. Omitted attributes remain unchanged.
Explicit null clears an optional field, including loanTypeKey. Omitting
purpose preserves the current purchase/refinance value.

Not the stage. Use PATCH /api/partner/loans/{id}/stage to move a loan
— it is a separate endpoint so that the loan and its history cannot drift
apart, and so a stage move can publish its own webhooks.