Function get_poster_gas 

pub fn get_poster_gas(
    tx_data: &[u8],
    l1_base_fee: U256,
    l2_base_fee: U256,
    _arbos_version: u64,
) -> (u64, u64)

Calculates the poster gas cost for a transaction’s calldata.

Returns (poster_gas, calldata_units) where:

• poster_gas: Gas that should be reserved for L1 posting costs
• calldata_units: The raw calldata units before price conversion