Fix: disable SIGINT propagation to child hived processes

Summary

Set propagate_sigint=False in BaseNode Settings to prevent accidental SIGINT propagation from the Python test process to child hived nodes.

Problem

By default, beekeepy uses propagate_sigint=True, causing all child hived processes to share the parent Python process's process group. When any SIGINT is sent to the Python test process (from pytest-xdist worker management, timeouts, or internal exception propagation), it propagates to ALL running hived child processes.

This caused flaky fork_tests failures in hive CI where WitnessNode4 would receive SIGINT during Phase 1 startup and crash.

Solution

Set propagate_sigint=False which creates a new process group for each hived process via os.setpgrp. This isolates child processes from parent signals while still allowing explicit SIGINT via send_signal() when closing nodes.

Related

Hive MR: hive!1758 (merged)