A session is registered before its worker is created, so an admin Kill or
the host drain could close it during the startup window and the worker
client that arrived afterwards was stored on the dead session, MarkReady
silently refused, SessionOpened recorded and the Closed session returned
to the caller with a live x86 worker nothing would ever stop.
AttachWorkerClient now returns false under _syncRoot when the session is
Closing/Closed/Faulted or disposed; the manager runs attach + MarkReady +
SessionOpened under the per-session close gate, discards (kills and
disposes) a refused worker client and fails the open with OpenFailed.
OpenRecorded gates the closed-side accounting so a never-opened session is
not counted as closed.
The open rollback goes through RemoveSessionAsync so per-session event
metrics are dropped like every other teardown.
Joseph Doherty
87f23d9712