Code Review

This pull request refactors the background thread creation logic by introducing a generic template class, AutomaticallyCreatedBackgroundThreadsImpl, to consolidate duplicate code from both gRPC and REST background thread implementations. However, the reviewer identified a critical issue where this generic implementation breaks REST pure background threads due to invalid default-construction of RestPureCompletionQueue and the omission of internal::CallContext setup. To resolve this, the reviewer suggested introducing a customizable QueueTraits template parameter to handle queue-specific initialization and execution logic.