internal/ethapi: use block GasLimit if gas is nil in DoEstimateGas

[darioush]

Hi. Prior to the refactor made in https://github.com/ethereum/go-ethereum/pull/28600/files, when args.Gas == nil, the block's GasLimit was used as the upper limit for gas estimation: https://github.com/ethereum/go-ethereum/pull/28600/files#diff-c426ecd2f7d247753b9ea8c1cc003f21fa412661c1f967d203d4edf8163da344L1208-L1216

After the mentioned PR, when args.Gas == nil, in case the default set in args.ToMessage (usually globalGasCap) is greater than the block's GasLimit, this can result in an estimation higher than the block GasLimit as it would be overwritten here: https://github.com/ethereum/go-ethereum/pull/28600/files#diff-66f4d55a856f5b04d4c6e1ee99582788b0510add7bd64c7332b948c2aeadbfdfR57-R60

This PR restores the previous behavior by setting the header's GasLimit in args prior to calling CallDefaults or ToMessage. Thanks for your consideration.

[karalabe]

The code you linked has the same behavior before and after though.

Original:

	if args.Gas != nil && uint64(*args.Gas) >= params.TxGas {
		hi = uint64(*args.Gas)

I.e. if the input already specified the global gas cap as the cap, then the block capping did not run. So that PR didn't change the behaviour.

[karalabe]

Ah, this is the culprit https://github.com/ethereum/go-ethereum/pull/28600/files#diff-c426ecd2f7d247753b9ea8c1cc003f21fa412661c1f967d203d4edf8163da344R1198, the PR added ToMessage, which did the change

[karalabe]

FWIW, this fix is definitely not good, because it just adds one more hack to undo a bad call. We need to pass the call args to esitmate without setting anything on the gas, like the original code intended.

[karalabe]

Going to close this PR because it's the wrong fix.