fuzz: Extend mini_miner fuzz coverage to max block weight
This is follow-up to #31384 which should be merged shortly. Only the last commit is new.
It expands the fuzz test to cover the full range of allow block sizes in BlockAssembler by utilizing the block_adjusted_max_weight option if necessary.
See also the brief discussion here for context: https://github.com/bitcoin/bitcoin/pull/31384/files#r1942039833