Per the Engine API spec, structure validation errors for PayloadAttributes
(e.g., missing withdrawals post-Shanghai, missing parentBeaconBlockRoot
post-Cancun) should return -32602 "Invalid params", not -38003.
This aligns with the spec requirement that parameter structure mismatches
return -32602, while -38003 is reserved for semantic validation errors
that occur after structure validation passes.
Fixes#8732