RT-2.16: fix graceful restart logic

[karthikeya-remilla]

Changes done:

1. fix timing logic in RT-2.16 to achieve desired GR traffic verification result
2. remove unused constants and trivial variable errors

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the IS-IS graceful restart test suite to improve reliability and correctness. Key changes include adjustments to timing and telemetry validation to better align with device behavior during restart cycles, as well as hardening the gNOI interaction logic to handle controller switchovers more gracefully by avoiding stale cached connections.

Highlights

• Graceful Restart Logic: Updated IS-IS graceful restart configuration to disable helper-only mode and adjusted timing logic to ensure accurate traffic verification.
• Controller Switchover Reliability: Improved gNOI client handling by dialing fresh connections during switchover and process killing to avoid issues with cached TCP channels.
• Test Robustness: Refined test telemetry validation, added missing network instance configuration, and improved traffic recovery verification steps.

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[OpenConfigBot]

Pull Request Functional Test Report for #5777 / 1e1e4f7

Virtual Devices

Device	Test	Test Documentation	Job	Raw Log
Arista cEOS		RT-2.16: IS-IS Graceful Restart Helper
Cisco 8000E		RT-2.16: IS-IS Graceful Restart Helper
Cisco XRd		RT-2.16: IS-IS Graceful Restart Helper
Juniper ncPTX		RT-2.16: IS-IS Graceful Restart Helper
Nokia SR Linux		RT-2.16: IS-IS Graceful Restart Helper
Openconfig Lemming		RT-2.16: IS-IS Graceful Restart Helper

Hardware Devices

Device	Test	Test Documentation	Raw Log
Arista 7808		RT-2.16: IS-IS Graceful Restart Helper
Cisco 8808		RT-2.16: IS-IS Graceful Restart Helper
Juniper PTX10008		RT-2.16: IS-IS Graceful Restart Helper
Nokia 7250 IXR-10e		RT-2.16: IS-IS Graceful Restart Helper

Help

[gemini-code-assist]

Code Review

This pull request refactors the IS-IS graceful restart and controller card switchover tests. Key changes include removing unused constants, dynamically dialing fresh gNOI clients to prevent issues with cached channels during switchovers, updating telemetry verification logic, and optimizing wait times. The feedback suggests removing a redundant static sleep in testGrHelper since the subsequent telemetry verification already dynamically awaits the UP state, aligning with the style guide's performance guidelines.