[cSONiC] Make show CLI work in docker-sonic-vs neighbors (no nested docker/rvtysh)

[securely1g]

Why I did it

In docker-sonic-vs containers used as cSONiC neighbors in the sonic-mgmt KVM testbed, FRR runs inside the same container — there is no nested bgp docker container and no rvtysh wrapper. Two sonic-utilities code paths assume the real-SONiC layout, so every relevant show command either leaks an error to stderr or fails outright:

1. show/main.py::get_routing_stack() runs a sudo docker ps ... one-liner at module import time (routing_stack = get_routing_stack() global). It therefore executes on every show invocation — even show ... --help — printing:

   sudo: docker: command not found
   

   The try/except does default to 'frr', so the value is correct, but the stderr leak is unconditional. The function's own # To be enhanced comment already flags this one-liner as undesirable.

2. utilities_common/bgp_util.py::run_bgp_show_command() uses RVTYSH_COMMAND = 'rvtysh', the routing-stack-aware wrapper that docker execs into the bgp container on real SONiC. In docker-sonic-vs the wrapper does not exist, so commands fail with:

   sudo: rvtysh: command not found
   

In a cSONiC neighbor only the plain vtysh binary exists (command -v rvtysh → not found; command -v vtysh → /usr/bin/vtysh).

Resolves #4632. This is the sonic-utilities counterpart to the cSONiC neighbor-environment gaps tracked in sonic-mgmt (e.g. #22647, #22648).

How I did it

• get_routing_stack(): short-circuit and return the default 'frr' when the docker binary is not present (shutil.which('docker') is None), so the bash one-liner never runs on single-container images and no stderr is leaked. Behavior on real SONiC (docker present) is unchanged.
• run_bgp_show_command(): select the vtysh wrapper at runtime — use rvtysh when it is present in PATH (real SONiC), otherwise fall back to the plain vtysh binary (docker-sonic-vs). No change to the argv/return contract.
• Added unit tests in tests/show_test.py (TestCsonicNeighborEnv) covering both docker-present/absent paths of get_routing_stack() and both rvtysh-present/absent paths of run_bgp_show_command().

How to verify it

Run the new unit tests:

pytest tests/show_test.py -k TestCsonicNeighborEnv

Manual verification on a live cSONiC neighbor (docker-sonic-vs) — before vs after this change:

Before:

# show ip --help        -> "sudo: docker: command not found" (leaked on stderr)
# show ip route         -> "sudo: rvtysh: command not found"
# show ip bgp summary   -> error / no output

After (verified):

# show ip --help        -> clean, no stderr leak
# show ip route         -> full FRR routing table
# show ip bgp summary   -> full IPv4 Unicast BGP summary (4 neighbors, real PfxRcd)

Which release branch to backport (provide reason below if selected)

Description for the changelog

Make the show CLI work in docker-sonic-vs (cSONiC) neighbor containers: skip the docker ps routing-stack probe when docker is absent, and fall back from rvtysh to vtysh when the rvtysh wrapper is not present.

[mssonicbld]

/azp run

[azure-pipelines]

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