fix: set gas floor for swaps to prevent out of gas

[owen-eth]

Describe your changes

Adds a 400k floor on the fastswap executor gasLimit to prevent OOG reverts on simple single-hop routes.

Why

For simple routes, Barter returns a small GasEstimation (~70k), and the formula × 2.5 + 135k/152k lands at ~310-330k — right at the median observed natural
gasUsed (316,980 across 29 on-chain samples). With the EIP-150 63/64 cascade through nested CALLs, inner calls OOG before the swap completes. Reproduced
against revm directly (anvil eth_simulateV1) and Tenderly archive.

Why a floor (not a multiplier bump)

Distribution is bimodal: simple routes ~272-320k natural gas, multi-hop ~600-700k. Barter's estimate already scales for the larger ones — only the simple
cluster hits the cliff. Floor at 400k touches only Barter estimates ≤ ~106k; everything above is unchanged.

Issue ticket number and link

Fixes # (issue)

Checklist before requesting a review

• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation