The callback marked an order success on any verify-ok result without comparing
the gateway-settled amount to the amount charged. SEP returns AffectiveAmount;
an underpayment or a replayed RefNum from a cheaper order would confirm the
expensive order. Now reject (status=failed, no activation) when the gateway
reports an amount that mismatches the stored amount_rials. Gateways that don't
report a settled amount (Mellat binds it server-side) skip the check.
MockGateway now echoes mock_amount so the guard is exercisable in tests.
Regression: tests/Payment/PaymentCallbackAmountTest (underpayment rejected,
matching amount succeeds).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
