[pre-MIP] "Valid while" precondition should be asserted and not take fee if it fails

[L-as]

zkApp commands can currently fail and still take fees if the preconditions fail. This is to allow for a simplified mempool design that does not need to take into account transaction dependencies.

However, transaction ordering can not affect whether the "valid while" precondition fails, thus, you can assert that it always succeeds, without changing the design of the mempool. The only change you need is to discard the transaction if the precondition fails on block production time, rather than trying to prove it.

[L-as]

@nholland94