- Add nil service guards to Register/Deregister/Renew in gRPC and HTTP
plugins to prevent nil pointer dereferences
- Remove unused log field from grpcPlugin and httpPlugin
- Prefix HTTP error messages with operation context (sd register/deregister/renew/get)
- Return gRPC Register error directly instead of logging + returning
- Add 33 unit tests covering all operations, nil guards, and error paths