Add tests for child process profiling

The test suite covers the get_child_pids C function with both recursive
and non-recursive enumeration, the is_python_process detection helper,
the ChildProcessMonitor lifecycle, and end-to-end CLI tests with
--children for both attach and run modes.

Tests use short-lived subprocesses with controlled lifecycles and
polling-based synchronization rather than fixed sleeps to keep runtime
reasonable. Resource cleanup is handled through reap_children and
explicit process termination in finally blocks.

Author: pablogsal