[action] [PR:22895] Fix test_add_rack flakiness by waiting for BGP convergence before DB comparison

[mssonicbld]

Description

\ est_add_rack\ was failing ~1% of PR and baseline runs with:
\
AssertionError: DB compare failed after adding T0 via generic patch updater
\\

Root Cause

In \generic_patch_add_t0(), the DB comparison (config-db, app-db, state-db) ran before BGP sessions established. After applying a JSON patch to add T0 config (BGP_NEIGHBOR, INTERFACE, PORT, etc.), BGP peers need time to converge and populate app-db route entries. The DB comparison would repeatedly fail its 5-minute retry window because app-db routes hadn't settled.

The BGP session check was placed after the DB comparison and used a bare \�ssert\ with no retry — so it never had a chance to gate the comparison.

Evidence (Kusto, last 30 days on master baseline+PR)

• 23 failures, ~90% with \DB compare failed after adding T0 via generic patch updater\
• Failures span random unrelated PRs (sonic-buildimage, sonic-mgmt) confirming it is flaky, not a regression
• Fail rate: ~0.6-1.0% on PRTest, ~0.6-2.1% on BaselineTest

Changes

• **\ ests/common/configlet/utils.py**: Add \is_bgp_session_established()\ helper that returns \True/False\ (compatible with \wait_until\ retry), with logging
• **\ ests/configlet/util/generic_patch.py**: Move BGP convergence check before DB comparison in \generic_patch_add_t0(), using \wait_until\ retry (up to 5 min). This ensures app-db routes are populated before comparing against baseline.

Before (broken ordering)

\
Apply patch → 60s pause → DB comparison (5min retry) ❌ → BGP check (no retry) ❌
\\

After (fixed ordering)

\
Apply patch → 60s pause → Wait BGP Established (5min retry) ✅ → DB comparison (5min retry) ✅
\\

[mssonicbld]

Original PR: #22895

[mssonicbld]

/azp run

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[StormLiangMS]

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[radha-danda]

/azpw run

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

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[mssonicbld]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[radha-danda]

/azpw run

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1072424:

✅Stage Test:

• Job impacted-area-kvmtest-t2 by Elastictest: retried.
• Job impacted-area-kvmtest-t1-lag by Elastictest: retried.
• Job impacted-area-kvmtest-t0 by Elastictest: retried.