With the eager pre-warm removed (PR #66783), slash.exec is the only spawn
path — and it runs on the RPC thread pool, so two concurrent worker-routed
commands on a fresh session could both see slash_worker=None and each fork
a full stdio-MCP-fleet worker (the _attach_worker race loser leaking
unclosed). Add a per-session spawn lock with a double-check, plus a
regression test racing two slash.exec calls through handle_request.
Also maps Ne0teric's contributor email.
