Fix long exec stdout truncated at websocket frame boundaries

WSClient.update() consumed at most one websocket frame per call, so
callers that read a channel after a single update() - for example the
peek_stdout()/read_stdout() pattern used by Airflow's pod_manager -
only received the first frame (32768 bytes) of a longer stream.

Keep reading frames within update() while more frames are already
waiting on the socket, so the channel buffers contain everything that
has been received when update() returns.

Signed-off-by: Jojin <jojin.kb@gmail.com>