Function finalize_runner_exact 

pub async fn finalize_runner_exact(
    runners: &Arc<RwLock<HashMap<String, AgentRunner>>>,
    session_id: &str,
    run_id: &str,
    result: &Result<(), AgentError>,
) -> bool

Finalize only the exact reserved runner.

Registration collisions can expose a stale caller after another entry point has become authoritative. That caller must release its own slot without ever terminalizing a newer runner stored under the same session id.