gnoi_shutdown_daemon: Replace docker exec gnoi_client with native gRPC calls

[sigabrtv1-ui]

Summary

The gnoi_shutdown_daemon currently makes gNOI System RPCs (Reboot HALT and RebootStatus) by shelling out to docker exec gnmi gnoi_client .... This creates an unnecessary dependency on the gnmi container being healthy, adds subprocess overhead, and relies on fragile stdout parsing.

Proposal

Replace the subprocess-based gnoi_client invocations with direct Python gRPC calls using generated protobuf stubs for the OpenConfig gNOI System service.

Key changes:

1. Vendor gNOI System proto stubs (system_pb2.py, system_pb2_grpc.py)
2. Add a lightweight GnoiClient wrapper for Reboot and RebootStatus RPCs
3. Refactor GnoiRebootHandler to use native gRPC instead of docker exec
4. Remove execute_command() subprocess helper (no longer needed)
5. Update tests to mock at the gRPC stub level

Benefits:

• No dependency on gnmi container being running
• Structured error handling via gRPC status codes
• No subprocess overhead or stdout parsing
• Smaller attack surface (no shell-out through Docker CLI)

Design Doc

See doc/gnoi-native-grpc-design.md in this repository for the full design.

Implementation Plan

Single PR covering all phases — proto stubs, client wrapper, daemon refactor, and test updates.