Handle IPC receive socket construction failures safely

[sylvesterkaczmarek]

Description

Fix the socket-construction failure path in SimpleReceiveExternalStepPy.

The node creates its listening socket inside a try block and catches OSError, but the handler unconditionally calls ls.close(). If socket.socket() itself raises before ls is assigned, the handler raises UnboundLocalError and masks the original recoverable socket failure.

Initialize ls to None and close it only when construction progressed far enough to create a socket. The node then returns False as intended and can retry on a later evaluation.

Validation

• focused regression test forces socket.socket() itself to raise OSError
• verifies compute() returns False without a secondary exception
• verifies no listening socket or URI state is committed
• normal bind/listen/accept behavior is unchanged
• production diff is three additions and one replacement