Modification of rail lockup prevented unless same epoch as settlement

[anorth]

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.

[Chaitu-Tatipamula]

I'd like to work on this issue. I've implemented a fix that:

1. Removes the overly strict isAccountLockupFullySettled check
2. Implements optimistic lockup changes with proper validation
3. Calculates exact differences between old and new lockup amounts
4. Only enforces restrictions when necessary (increases)
5. Safely handles decreases without additional restrictions

Could you please assign this issue to me @anorth @rjan90 @aarshkshah1992 ?

[rjan90]

I have assigned the ticket to you @Chaitu-Tatipamula