fix: complete payload builder gas overflow error propagation

[crazywriter1]

Summary

Completes the defensive gas accounting started in #42 (reopen of #24). That PR replaced saturating_add with checked_add, but left two .expect("total gas shouldn't overflow") calls that could still panic the payload builder. This change applies the error-handling pattern requested by @atiwari-circle in the #24 review.

Changes

crates/execution-payload/src/payload.rs

1. Pre-execution capacity check — cumulative_gas_used.checked_add(pool_tx.gas_limit()).is_none_or(|total| total > block_gas_limit)
   If addition overflows (not practically reachable — both values are bounded by block_gas_limit), the transaction is treated as not fitting and marked invalid via ExceedsGasLimit, instead of panicking.

2. Post-execution cumulative update — checked_add(gas_used) now returns PayloadBuilderError on overflow instead of .expect(). A silent clamp or panic would let the builder continue past safe accounting; overflow now fails the build cleanly.

Context

• fix: use checked arithmetic for cumulative gas accounting in payload builder #24 — original PR; review noted saturating_add on the cumulative update was wrong
• fix: use checked arithmetic for cumulative gas accounting in payload builder #42 — merged checked_add but retained .expect() at both sites
• Overflow remains theoretically unreachable in production (block_gas_limit ~30M vs u64::MAX), but checked_add with explicit propagation is the semantically correct primitive

Testing

• cargo fmt -p arc-execution-payload -- --check
• cargo clippy -p arc-execution-payload --all-targets -- -D warnings
• cargo test -p arc-execution-payload — 25 passed

[ZhiyuCircle]

Hi @crazywriter1,

Thank you for your interest in contributing to Arc Node, and apologies for the delay in getting back to this PR.

We're closing out the pull request backlog that predates our current contribution policy. This PR is being closed because the number it references is a pull request, not an issue. All PRs must reference an existing issue using the format Closes: #XXX, and the author must be assigned to that issue before the PR is opened.

This is not a judgement on the change itself. If you'd still like to land it:

1. Open an issue describing the problem, or find the existing one
2. Comment on the issue requesting assignment, and wait for maintainer approval
3. Open a fresh PR once you have been assigned

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