Cancels an existing loan application that was not charged yet.
This operation updates the application status to CANCELLED and prevents further processing.
The response includes a cancellation_reason field. When no reason is provided in the request body, the value defaults to PARTNER_REQUESTED.

Auto-expiry: applications in APPLICATION_COMPLETED status whose OTP validity window has passed are automatically cancelled by the platform with cancellation_reason: OFFER_EXPIRED. No outbound webhook is sent for cancellations (only APPLICATION_DECLINED and FUNDED trigger outbound webhooks). Partners can detect an auto-expired application by polling GET /application/{id}/status and reading the cancellation_reason field.