Function stop_one_agent_block 

pub(crate) async fn stop_one_agent_block(
    state: &AppState,
    block_id: &str,
    signal: Option<&str>,
    stop_background: Option<bool>,
) -> Result<AgentStopResult, String>

Core agent.stop logic, factored out so fleet.bulk-stop (server/app_api/fleet.rs) can loop it per target instead of duplicating the controller-lookup/stop/status-read sequence.