Function do_forum_mark_read 

pub async fn do_forum_mark_read(
    ctx: &CoreCtx,
    token: &str,
    topic_id: i64,
    last_read_post_id: Option<i64>,
) -> Result<PlatformMessage>

forum read core — mark a topic read (auth). last_read_post_id defaults to the topic’s newest post when None.