Function spawn_on_lsp_runtime 

pub fn spawn_on_lsp_runtime<F>(future: F) -> JoinHandle<<F as Future>::Output> 
where
    F: Future + Send + 'static,
    <F as Future>::Output: Send + 'static,

Phase 5.5.LSP.1: spawn a fire-and-forget future on the shared LSP runtime. Used by the App’s + host’s per-feature LSP dispatchers so the request awaits the actor’s response off the main UI thread; the result flows back through a per- feature mpsc channel that the App drains before each draw.

Returning a JoinHandle lets the caller cancel by dropping it – though for LSP cooperative cancellation runs through the CancellationToken plumbed into the typed wrappers, so the handle is mostly informational.