Function send_unseen_mempool_txs 

async fn send_unseen_mempool_txs<Mempool: MempoolService>(
    mempool: &Mempool,
    response_sender: &Sender<Result<RawTransaction, Status>>,
    sent_txids: &mut HashSet<Hash>,
) -> bool

Sends every mempool transaction that isn’t already in sent_txids, adding the ones it sends, and returns false if the client went away or the mempool query failed.

Used for the initial snapshot, and again whenever the change subscription lags and the stream would otherwise silently omit the dropped transactions.