Function close_on_exit 

pub(crate) fn close_on_exit(tab_id: String, block_id: String)

Trigger the installed close-on-exit handler for (tab_id, block_id), if one has been installed. Fire-and-forget: the handler owns spawning whatever async work it needs (the saga call is async; this function itself is not). A missing handler (astronomically early in boot, before main.rs finishes wiring AppState — no real caller can reach this before then, since nothing spawns a PTY that early) just skips closing; the pane sits inert the same way it did before this feature existed, not a hard failure.