fix(test): stop ProcessTreeProbe from leaking orphaned fake-codex processes

[chihsuan]

Context

The turn-timeout side-output test used a ProcessTreeProbe double that recorded the cleanup call but never ran it, so each test run orphaned a fake-codex busy loop that pinned a CPU core indefinitely.

TL;DR

Make the test's process-tree cleanup double actually terminate the spawned process tree instead of no-op'ing it.

Summary

• ProcessTreeProbe.terminate_port_descendants/1 now delegates to the real SymphonyElixir.ProcessTree after sending its verification message.
• Stops the bash -> /bin/sh fake-codex busy loop (while true; sleep 0.01) from leaking as an orphan on every run of the turn-timeout side-output test.
• Keeps the existing {:process_tree_cleanup, true} assertion intact; only the dropped side effect is restored.

Alternatives

• Add an on_exit sweep to kill stray fake-codex processes: treats the symptom, not the missing cleanup in the double.
• Slow the fake-codex loop (sleep 0.5): reduces but does not remove the leak.

Test Plan

• make all
• mix test test/symphony_elixir/app_server_test.exs:1864 then pgrep -fl symphony-elixir-app-server-turn-timeout returns no orphaned processes