[armadavalor]

Unread batches queued in an Unbatcher hold pooled NetworkWriterPooled instances. They were never returned when the connection went away, so the pool lost one writer (with its expanded buffer) per such disconnect.

Server: NetworkConnection.Cleanup() only cleared the send-side batchers. NetworkConnectionToClient now overrides Cleanup() and clears unbatcher. OnTransportDisconnected always calls Cleanup(), so this also covers the UnpackAndInvoke failure path, which calls Disconnect() and returns without clearing.

Client: NetworkClient.Initialize() and Shutdown() replaced unbatcher with a new instance and dropped the old queue. They now call Clear() first.

Verification: I added Cleanup_ClearsUnbatcher to NetworkConnectionToClientTests. I could not run it because I have no Unity Editor here, so this is untested in Unity. The change is three Clear() calls on an existing method, and the diff is 25 added lines.

Closes #4125