RT-5.1: Missing L2 MTU & subinterface neighbor origin telemetry paths in singleton_test README.md

[devansh-lodha]

Describe the bug
The test specification in feature/interface/singleton/otg_tests/singleton_test/README.md omits three OpenConfig telemetry state paths that are actively configured and verified in singleton_test.go:

1. /interfaces/interface/state/mtu
2. /interfaces/interface/subinterfaces/subinterface/ipv4/neighbors/neighbor/state/origin
3. /interfaces/interface/subinterfaces/subinterface/ipv6/neighbors/neighbor/state/origin

Additionally, the legacy README.md lacked a valid ## Canonical OC block, causing validate_readme_canonicalocspec to fail.

To Reproduce
Steps to reproduce the behavior:

1. Inspect feature/interface/singleton/otg_tests/singleton_test/singleton_test.go:
   • Line 120 (configInterfaceDUT): i.Mtu = ygot.Uint16(tc.mtu + 14)
   • Line 289 (verifyInterfaceDUT): gnmi.Get(t, tc.dut, dis4np.Origin().State())
   • Line 295 (verifyInterfaceDUT): gnmi.Get(t, tc.dut, dis6np.Origin().State())
2. Run go run ./tools/validate_readme_canonicalocspec -alsologtostderr feature/interface/singleton/otg_tests/singleton_test/README.md.
3. Observe missing Canonical OC error.

Expected behavior
README.md should register all verified telemetry state paths, include a valid RFC 7951 Canonical OC block, and pass validate_readme_canonicalocspec and validate_readme_ocpaths.

Logs
N/A (Specification gap & validator audit)

Network device platform:

• Device: Universal / OTG Testbed
• OS: OpenConfig / Ondatra
• Version of the test: main branch (singleton_test)