Prevent partial JSON-RPC frames in async transport

[jserv]

Summary

send_async() performs two separate awaited writes (JSON body, then newline). If the first succeeds but the second fails or the task is cancelled between awaits, the client receives a JSON body with no trailing newline — an incomplete NDJSON frame that breaks the parser.

Details

• The sync transport uses a single writeln! on a BufWriter<StdoutLock>, so it doesn't have this problem.
• Fix: serialize to String, push '\n', and call write_all once. Treat any write error as fatal.
• Scope: transport_async.rs::send_async() only.