robustness: 20221 reproduction

[henrybear327]

Targeting bug fix #20281

The idea is that we have a watch that was connected to a peer A, and have received event there. Due to cluster client switching the connection to a different peer, with the blackholeUntilSnapshot failpoint enabled, by chance it would connect to a peer that is blackholed and lags behind. In this case, due to the bug, we could get a revision that is in the past, as seen in the log.

    logger.go:146: 2026-03-11T16:11:25.397Z     ERROR   Broke watch guarantee   {"guarantee": "resumable", "client": 20, "request": {"Key":"key","Revision":1593,"WithPrefix":true,"WithProgressNotify":true,"WithPrevKV":true}, "got-event": {"Type":"put-operation","Key":"key76","Value":{"Value":"1981"},"Revision":1571,"PrevValue":{"Value":{"Value":"1976"},"ModRevision":1567,"Version":23}}, "want-event": {"Type":"put-operation","Key":"key80","Value":{"Value":"2010"},"Revision":1593}}

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@henrybear327: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-robustness-amd64	f8ad38d	link	true	/test pull-etcd-robustness-amd64

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.43%. Comparing base (feea852) to head (f8ad38d).

⚠️ Current head f8ad38d differs from pull request most recent head 7b00e69

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Additional details and impacted files

see 43 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21472      +/-   ##
==========================================
- Coverage   68.48%   68.43%   -0.06%     
==========================================
  Files         432      428       -4     
  Lines       35414    35291     -123     
==========================================
- Hits        24253    24151     -102     
+ Misses       9758     9740      -18     
+ Partials     1403     1400       -3     

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