Refund helper contract

The caller has already validated that all three inputs are integer cents,
paid_cents >= 0, and 0 <= refunded_cents <= paid_cents.
The helper must validate request_cents, which can be negative.

1. Reject a negative request with ValueError.
2. Reject a request larger than paid_cents - refunded_cents with ValueError.
3. Allow zero, returning the available balance unchanged.
4. Allow a request equal to the available balance, returning zero.
5. Return the available balance minus the request for all other valid requests.

This function does not contact a payment service or mutate stored data.
Do not add currency conversion, floating-point arithmetic, or input parsing.