Implements the foundational gRPC client class with:
- Lazy ClientChannel initialization with proper credentials
- DeliveryServiceClient lazy initialization
- Authentication via Bearer token in gRPC metadata
- CallOptions builder with token injection
- gRPC error to ApiError mapping (status codes -> HTTP equivalents)
- Token refresh on UNAUTHENTICATED errors (single retry)
- Proper channel shutdown/terminate methods
The core _executeWithAuth and _executeCommandWithAuth methods provide
the foundation for query and command methods in subsequent subtasks.
Co-Authored-By: Claude <noreply@anthropic.com>
