Function commit 

pub fn commit(
    session: &mut Session,
    env: &Envelope,
    response: ModeResponse,
    now_ms: i64,
) -> SessionState

Commit a validated message into the session: consume the dedup slot, record participant activity, and apply the mode response. Returns the resulting session state.

A durable consumer MUST call this only after the message has been durably recorded, so a failed write never consumes a dedup slot. Because nothing here mutates the session until validation has already succeeded, a rejected message likewise leaves seen_message_ids untouched.