A receive right after all([...]) returns null with a message pending #1021

Problem

In embedded workflow code, inbox()->receive() / receiveOne() called right after all([...]) returns no message while one is pending on the stream. General's interim review of #1017 found this. The same happens on main before #1017, so it predates that PR.

To investigate

The likely cause is the step the receive sees after all(). A parallel group takes several step positions, and visibleSequence after all() may not be the position the receive is replayed or recorded at. It may also be that the replay guard or recorded-consumption lookup keys on a sequence that the group's children share. Reproduce with a workflow that awaits a signal, runs all([activity(...), activity(...)]), then calls $this->inbox('chat')->receiveOne() with one message pending. Check whether it returns null on the first execution, only on replay, or both.

Expected

A receive after all([...]) behaves like a receive after a single step: on the first execution it consumes the pending message, and on replay it returns that message.