fix: Complete a pending identify or start with an error when the client closes

[jsonbailey]

Summary

If close() was called while identify() or start() was still waiting for flag data, the returned future never completed. Code that awaited it with no timeout waited forever.

• When the client closes, a pending identify() completes with IdentifyError ("The client was closed before the identify completed."), and a pending start() completes with false.
• Identifies and starts that have already completed are not affected. The start identify still can never be superseded.
• The IdentifyError, close(), identify() and start() docs describe the close behavior.

[jsonbailey]

Cross-SDK note, for team discussion: iOS and Android both leave a pending identify waiting after close. On iOS, internalClose goes offline without calling the completion. On Android, ConnectivityManager.shutDown stops the data source with a no-op callback. Flutter completes it with IdentifyError("The client was closed before the identify completed."), so that code awaiting identify without a timeout does not wait forever.