fix mock NMR deregister race.

[Shankar Seal (shankarseal)]

Summary

Fixes #318 an NMR client deregister can race with an incomplete NMR provider registration.

• Deregistering modules always return STATUS_PENDING, matching Windows NMR behavior.
• After attach succeeds, the binding is marked attached. If either module is already deregistering, it is marked LateBind and unbound immediately.
• pending_bind_ops tracks accepted attach operations that have not completed. Each module also tracks all successfully attached bindings.
• A module cannot be disposed until both pending bind operations are complete and its attached-binding list is empty.
• Each binding starts unbind only once. After both detach callbacks complete, it is removed from both module lists and the global binding map.
• add a concurrent register/deregister smoke test that loops provider and client registration/deregistration and prints executed iteration counts

Testing

• run usersim_tests nmr tests.