Function fetch_replies 

async fn fetch_replies(
    api: &dyn SlackApi,
    workspace: &str,
    channel: &str,
    ts: &str,
    oldest: Option<&str>,
) -> Result<Vec<SlackHistoryMessage>, SlackError>

Every reply in a thread, following the cursor. Dedupe is the caller’s: Slack repeats the parent on every page, so the caller takes the newest by comparing ts rather than by taking the last one it read.