Function read_thread 

pub async fn read_thread(
    api: &dyn SlackApi,
    channel: &str,
    ts: &str,
    limit: u32,
) -> Result<Vec<SlackHistoryMessage>, SlackError>

Every reply in a thread, the parent first, for a caller reading the thread rather than advancing a cursor. Slack repeats the parent at the top of every page, so the result is deduped by ts; the first occurrence wins, which keeps the parent where the contract puts it. limit bounds the rows returned, and the walk stops as soon as it has them.