When kill_process() sends SIGTERM, both it and the reader thread race
to call _move_to_finished() — kill_process sets exit_code=-15 and
enqueues a notification, then the reader thread's process.wait()
returns with exit_code=143 (128+SIGTERM) and enqueues a second one.
Fix: make _move_to_finished() idempotent by tracking whether the
session was actually removed from _running. The second call sees it
was already moved and skips the completion_queue.put().
Adds regression test: test_move_to_finished_idempotent_no_duplicate
c8e4dcf412