_run_subpolls is a self-limiting background loop the coordinator already
cancels and recreates every refresh cycle, but scheduling it with
async_create_task ties it into HA's startup/shutdown task tracking
anyway. A subpoll cycle in flight (up to ~27s) then delays both.
async_create_background_task is HA's supported API for exactly this
case -- a task the integration owns and manages the lifecycle of.