[gnoi] PTF-free native gNOI test suite (sonic_grpc.GnoiClient)

[hdwhdw]

Tracking issue for a new PTF-free native gNOI test suite in sonic-mgmt.

Goal

Add a tests/gnoi/ suite that drives gNOI the "API way": an in-process Python
gRPC client (sonic_grpc.gnoi.GnoiClient) running in the sonic-mgmt container
talks mTLS gRPC directly to the DUT gNOI server on the management IP — no PTF
hop, no in-repo stub generation. The client and its vendored gNOI proto stubs
are provided by a standalone sonic-grpc wheel.

Motivation

Today gNOI tests (tests/gnmi/test_gnoi_*.py) shell out to a gRPC CLI
(grpcurl / py_gnmicli.py) on the PTF container. That couples every gNOI test
to PTF, uses stringly-typed dynamic messages, and hides real gRPC status codes
behind text parsing. A native typed client removes the PTF hop and surfaces
grpc.RpcError status codes directly.

Work items

• sonic-grpc wheel providing GnoiClient + vendored gNOI stubs
  (System, File) — sonic-buildimage (wired into docker-sonic-mgmt).
• tests/gnoi/ v1 PoC: System.Time + File.Stat over native mTLS,
  validated on a KVM VS testbed — sonic-mgmt (depends on the wheel).
• Follow-ups: broader gNOI surface (OS, Healthz, …), SmartSwitch DPU
  routing, migrate existing tests/gnmi/test_gnoi_*.py.

Status

v1 PoC implemented and validated on a KVM VS testbed (vms-kvm-t0): both
System.Time and File.Stat pass over the native mTLS client. PRs linked
below.

[hdwhdw]

PRs:

• [sonic-grpc]: Add gNOI Python gRPC client framework wheel sonic-buildimage#28341 — sonic-grpc wheel (GnoiClient + vendored System/File stubs), wired into docker-sonic-mgmt. (merge first)
• [gnoi]: Add PTF-free native gNOI test suite (System.Time, File.Stat) #26040 — tests/gnoi/ v1 (System.Time + File.Stat), depends on the wheel above. (draft until the wheel ships in the image)