Fix race condition with cancellation tokens in Read/Flush operations (#59090)

Fix race condition with cancellation tokens in Read/Flush operations (#59090)

* Fix race condition with cancellation tokens in Read/Write operations
- There's a tight race where UnsafeRegister can fire inline and then ReadAsync never completes. This is because the cancellation token property has not been assigned yet so the callback noops. This change checks the result of the UnsafeRegister operation to see if ran synchronously and throws if it did. We also move any state transitions to after these checks to make sure the PipeAwaitable state doesn't change before throwing.

* Add a debug assert to make sure there's no state change.

* Add if debug

* More if DEBUG