In particular ensuring we use `ctx._pld_rx.recv_msg_nowait()` from
`.receive_nowait()` (which is called from `.aclose()`) such that we
ALWAYS (can) set the surrounding `Context._result/._outcome_msg` attrs
on reception of a final `Return`!!
This fixes a final stream-teardown-race-condition-bug where prior we
normally didn't set the `Context._result/._outcome_msg` in such cases.
This is **precisely because** `.receive_nowait()` only returns the
`pld` and when called from `.aclose()` this value is discarded, meaning
so is its boxing `Return` despite consuming it from the underlying
`._rx_chan`..
Longer term this should be solved differently by ensuring such races
cases are handled at a higher scope like inside `Context._deliver_msg()`
or the `Portal.open_context()` enter/exit blocks? Add a detailed warning
note and todos for all this around the special case block!
5497401920