[action] [PR:21939] Fix/nonlinear high nexthop dataplane downtime

[mssonicbld]

Description of PR

Summary:
Fixes # (issue)
This PR fixes excessively high dataplane downtime attributed to nexthop behavior in the high‑BGP test scenarios

Nexthop handling in the test logic caused downtime measurements to stay high and inconsistent. This PR corrects nexthop‑related announcement, and verification so that:

• Traffic is always tested towards valid, expected nexthops,
• Stale or mis‑mapped nexthops no longer inflate the observed downtime,
• Downtime better reflects the actual behavior.

The fix put out in PR #20842 now also fixes the recently found issue where the failed nexthop_group_member_scale pollutes the test environment for future re-runs of the entire testbed.

Dependency:

• Depends on the fixes introduced in:
• PR #21936

Type of change

• [ x ] Bug fix
• Testbed and Framework(new/improvement)
• New Test case
• Skipped for non-supported platforms
• Test case improvement

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505

Approach

What is the motivation for this PR?

• Measured dataplane downtime remained unexpectedly high when:
• The number of nexthops increased,
• The test exercised different nexthop sets or ECMP groups.
• Downtime spikes appeared that did not match the BGP session and route programming timelines.

How did you do it?

• A fresh clean ptf dataplane environment for the nexthop group member scale similar to the PR #21936
• Uses the bulk reannouncement of the starting state as per the fix introduced by PR #20842

How did you verify/test it?

• Ran the high‑BGP convergence, flap, nexthop group member scale tests end‑to‑end with the nexthop fixes applied on:

• Topology: t0-isolated-d2u510s2

• Platform: Broadcom Arista-7060X6-64PE-B-C512S2

• Verified that the dataplane downtime does not fail the expected the MAX_DOWNTIME_NEXTHOP_GROUP_MEMBER_CHANGE of 30 seconds.

Dataplane Downtime results before: 63 seconds > MAX_DOWNTIME_NEXTHOP_GROUP_MEMBER_CHANGE
Dataplane Downtime results now:
Shutdown Phase - 0.11 seconds as expected
Startup Phase - 0.14 seconds as expected

Also fixes the recently found issue where the failed nexthop group member scale pollutes the FIB on the switch for future re runs of the testbed.

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[mssonicbld]

Original PR: #21939

[mssonicbld]

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[radha-danda]

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[mssonicbld]

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