Modification of rail lockup prevented unless same epoch as settlement

Modifying a rail lockup requires `isAccountLockupFullySettled`, but this is a bit confusingly named. It checks that the last settlement epoch is this epoch, but that's much stricter than whether the fixed lockup is settled. The error message "insufficient funds to cover the current lockup" is not necessarily correct.

https://github.com/FilOzone/filecoin-services-payments/blob/182b414d31005e6002c43eddfe7c4db344d1cdce/src/Payments.sol#L592-L603

Consider proceeding to optimistically change the lockup, and revert if necessary after calculating the actual funds needed.

Start in src/Payments.sol around lines 592-603 and trace how rail lockup modification uses isAccountLockupFullySettled. Review the settlement and funds-needed calculations before deciding how the lockup change should proceed. Done means valid modifications are not blocked solely by the settlement epoch, and insufficient-funds errors describe the actual requirement.

Written by the indexing model from the issue text.