fix: use checked arithmetic for cumulative gas accounting in payload builder

[Himess]

Summary

Use checked arithmetic for cumulative gas accounting in the payload builder to prevent potential overflow on u64 addition.

Two instances in arc_ethereum_payload:

• Line 579: cumulative_gas_used + pool_tx.gas_limit() — replaced with checked_add + is_none_or to reject the transaction if the addition overflows
• Line 660: cumulative_gas_used += gas_used — replaced with saturating_add to prevent wrapping

This follows the same defensive arithmetic pattern applied to reward_beneficiary in #21, where u128 multiplication was replaced with U256 arithmetic to prevent overflow.

[atiwari-circle]

Overflow here is not practically reachable -- cumulative_gas_used and pool_tx.gas_limit() are both bounded by block_gas_limit (~30M), nowhere near u64::MAX (~18.4 exagas). The checked_add at line 579 is fine if you want to keep it as defensive hardening, but saturating_add at line 660 is the wrong choice -- it would silently clamp at u64::MAX instead of failing the block build. Use checked_add with error propagation there, or leave it as plain addition.

[Himess]

Thanks for the review @atiwari-circle. Agreed, replaced saturating_add with checked_add + PayloadBuilderError propagation in 4755edd, so an overflow (still unreachable in practice) would now fail the build cleanly instead of silently clamping. Kept checked_add at line 579 as defensive hardening.

[ZhiyuCircle]

Hi @Himess ,

Apologies for the unexpected close. This PR was auto-closed as a side-effect of an update to the main branch in circlefin/arc-node. GitHub closes PRs when their base commit no longer exists on the target branch, which is why your contribution ended up here.

Your change is still very much welcome. To restore it, please rebase your branch onto the new main and open a fresh PR:

git fetch origin
git rebase origin/main
# resolve any conflicts if they come up
git push --force-with-lease

Then open a new PR against main — feel free to link back to this one for context, and we'll prioritize review. Thanks for contributing to Arc, and sorry for the inconvenience.