mvcc: add watch failpoints for robustness testing

[darshil929]

This adds three gofail failpoints to the etcd server watch code so the robustness tests can inject slowness into the paths listed in issue #18681: watcher synchronization, victim resynchronization, and progress notification. All three sit in server/storage/mvcc/watchable_store.go, right before each function takes the store lock, so an injected sleep delays the operation without holding the lock during the sleep.

• beforeSyncWatchers in syncWatchers, the batch that catches up unsynced watchers from the backend. Slowness here simulates watchers on old revisions staying behind under load.
• beforeMoveVictims in moveVictims, which retries delivery to watchers whose channels blocked. Slowness here extends the backpressure window while the system is already struggling.
• beforeProgressIfSync in progressIfSync, which both the periodic progress notify ticker and manual progress requests flow through. Slowness here stresses clients that depend on progress guarantees, like the case in Watch on future revision might receive old events or notifications, happens when client connects to multiple members. #20221

Each failpoint is registered as a gofailSleepAndDeactivate for the exploratory scenarios, following the same pattern as beforeSendWatchResponse from #17680. No build changes are needed since gofail-enable already instruments server/storage/mvcc.

Validation:

• Built with make gofail-enable and make build, confirmed all three failpoints appear on the GOFAIL_HTTP endpoint.
• Manually activated beforeSyncWatchers=sleep(1000) on a live member, verified watch delivery stays correct and the endpoint stays healthy.
• Ran TestRobustnessExploratory/KubernetesLowTraffic with the new failpoints in the pool, which passes.

Fixes #18681

[kubernetes-prow]

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[kubernetes-prow]

Hi @darshil929. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[serathius]

/ok-to-test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.65%. Comparing base (b9ba05c) to head (2403dcf).

Additional details and impacted files

Files with missing lines	Coverage Δ
server/storage/mvcc/watchable_store.go	92.88% <ø> (ø)

... and 12 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #22044      +/-   ##
==========================================
- Coverage   69.66%   69.65%   -0.02%     
==========================================
  Files         449      449              
  Lines       38177    38177              
==========================================
- Hits        26596    26591       -5     
- Misses      10149    10153       +4     
- Partials     1432     1433       +1     

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[darshil929]

/retest

[serathius]

/retest

[henrybear327]

/retest