Function decimal256_to_be32 

pub fn decimal256_to_be32(col: &Decimal256Array) -> BinaryArray

Encodes a Decimal256 column as fixed 32-byte big-endian two’s-complement binary.

Matches the wire format produced by evm-decode’s large_int_as_binary for int256 and uint256. Unlike u256_column_to_binary, this preserves the bit pattern of negative values (no i256 → U256 conversion), so it works for both signed and unsigned 256-bit integers.