Concurrent sock.sendMessage() calls on a single Baileys socket can cause
the WhatsApp protocol-level routing to misdeliver messages — responses
intended for one chat appear in another.
Add a promise-based send queue that serialises all sendMessage() calls
across concurrent HTTP /send, /edit, and /send-media handlers so only
one send is in-flight at a time.
Includes unit tests for queue ordering, error isolation, timeout
propagation, and single-consumer concurrency semantics, plus an
integration check that the queue is wired into sendWithTimeout.
