Function fill_tx 

async fn fill_tx<N: Network, P: Provider<N>>(
    provider: &P,
    tx: &mut N::TransactionRequest,
    from: Address,
    chain: Chain,
) -> Result<()>
where
    N::TransactionRequest: FoundryTransactionBuilder<N>,

Fills from, chain_id, nonce, fees, and gas limit on a transaction request for the browser wallet path. Mirrors the filling logic in the shared tx builder but operates on a pre-built transaction request from the sol! macro rather than through the builder pipeline. Only fills fields that haven’t already been set by the user.