Function serve_on 

pub fn serve_on<C, L>(
    connection: Connection,
    language: L,
    documents: Documents<C>,
) -> Result<(), Box<dyn Error + Send + Sync>>
where
    C: Clone + PartialEq + Send + 'static,
    L: Language<C>,

Runs a full server lifecycle over an existing connection: the initialize handshake, then the message loop until exit.

Use this instead of serve when you own the transport (TCP, an in-process Connection::memory pair, tests). The caller is responsible for any I/O threads; serve_on returns once the client sends shutdown

• exit.