fix(EL): payload builder no longer panics on cumulative-gas checked_add overflow

[UzairRehman4]

Summary

Both cumulative-gas accounting sites in crates/execution-payload/src/payload.rs used checked_add(...).expect("total gas shouldn't overflow"), which aborts the process on overflow instead of failing through the structured error path this function already uses everywhere else.

• Pre-execution capacity check: an overflowing cumulative_gas_used + pool_tx.gas_limit() can never fit in a bounded block, so it's now treated the same as "doesn't fit" — the tx is marked invalid via ExceedsGasLimit, evicted from the pool if permanently un-includable, and the loop continues. No panic.
• Post-execution cumulative update: now returns a PayloadBuilderError (wrapping a new CumulativeGasOverflowError, following the existing UnprocessableTransactionError pattern in builder.rs) via ? instead of panicking, so the build fails cleanly through the existing Result path.

No hardfork gate — this is builder-local error handling only, as scoped in the issue.

Fixes #452

Acceptance criteria (from the issue)

• Both .expect("total gas shouldn't overflow") call sites in payload.rs are removed
• Overflow / non-fit paths return errors (no panic)
• cargo test -p arc-execution-payload passes — I was not able to run this locally (no Rust toolchain available in my environment). I reviewed the change carefully against the existing code patterns (the ExceedsGasLimit branch is reused unchanged, and CumulativeGasOverflowError mirrors UnprocessableTransactionError exactly), but please run the test suite / CI before merging and let me know if anything needs adjusting.

Test plan

• Manual code review against the existing patterns in this file (see above)
• CI / cargo test -p arc-execution-payload should be run by a maintainer or CI, since I couldn't run it locally

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[github-actions]

Hi @UzairRehman4,

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