When the remote side disconnects while send() is in flight, send() was
setting _closing=True. This prevented the receive loop from firing
on_client_disconnected, causing the pipeline to hang waiting for a
disconnect signal that never came.
The fix removes _closing from send() (that flag means we initiated the
close) and instead checks Starlette application_state in _can_send()
to suppress subsequent sends after a failure.
Fixes#3912
