gh-139373: Fix asyncio Process.communicate() losing output when cancelled (#154223)

gh-139373: Fix asyncio Process.communicate() losing output when cancelled (#154223)

Output already read when communicate() is cancelled (e.g. by a
wait_for() timeout) is now retained on the Process and returned by a
subsequent communicate() call, matching subprocess.Popen.communicate()
behaviour on timeout. Passing input to a communicate() call following a
cancelled one now raises ValueError.