Function send_l1_to_l2_message 

pub async fn send_l1_to_l2_message(
    eth_client: &EthClient,
    inbox_address: &str,
    recipient: &AztecAddress,
    rollup_version: u64,
    content: &Fr,
    secret_hash: &Fr,
) -> Result<L1ToL2MessageSentResult, Error>

Send an L1→L2 message via the Inbox contract.

Returns the message hash and global leaf index from the MessageSent event.