api: set Stop field on ExponentialBackOff in LifetimeWatcher

[raman1236]

Description

Fixes a regression introduced in #26868 where the backoff package upgrade from v3 to v4 broke error propagation in the LifetimeWatcher.

Root Cause

When the ExponentialBackOff struct was upgraded from backoff v3 to v4, it was directly initialized instead of using backoff.NewExponentialBackOff(). In v4, the Stop field on ExponentialBackOff has a zero value of 0 (a time.Duration), but the sentinel value backoff.Stop is -1s. The constructor NewExponentialBackOff() sets Stop: backoff.Stop, but the direct struct initialization in lifetime_watcher.go omitted this field.

This means that when MaxElapsedTime is exceeded during persistent renewal failures, NextBackOff() returns 0 (the struct's Stop field value) instead of backoff.Stop (-1s). The subsequent check:

if sleepDuration == backoff.Stop {
    return err
}

never matches, so doneCh never receives the renewal error. The watcher eventually exits via the grace period check returning nil, silently losing the error.

Fix

Add Stop: backoff.Stop to the ExponentialBackOff struct literal so that NextBackOff() correctly returns backoff.Stop when the maximum elapsed time is exceeded. This is a one-line change.

Testing

• Added TestLifetimeWatcherErrorBackoffStops regression test that creates a LifetimeWatcher with a short lease and persistent renewal failures, verifying the watcher terminates promptly instead of getting stuck
• All existing TestLifetimeWatcher, TestRenewer_NewRenewer, and TestCalcSleepPeriod tests continue to pass

$ go test -run "TestLifetimeWatcher|TestCalcSleepPeriod|TestRenewer" -v ./api/
--- PASS: TestRenewer_NewRenewer (0.00s)
--- PASS: TestLifetimeWatcherErrorBackoffStops (0.96s)
--- PASS: TestLifetimeWatcher (71.34s)
    --- PASS: TestLifetimeWatcher/no_error (0.00s)
    --- PASS: TestLifetimeWatcher/short_increment_duration (0.00s)
    --- PASS: TestLifetimeWatcher/one_error (0.61s)
    --- PASS: TestLifetimeWatcher/many_errors (8.06s)
    --- PASS: TestLifetimeWatcher/only_errors (8.58s)
    --- PASS: TestLifetimeWatcher/negative_lease_duration (0.00s)
    --- PASS: TestLifetimeWatcher/permission_denied_error (54.10s)
PASS

Fixes #28611

[vercel]

@ramanvasi is attempting to deploy a commit to the HashiCorp Team on Vercel.

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[hashicorp-cla-app]

All committers have signed the CLA.

[raman1236]

Friendly ping — this PR fixes a subtle regression where ExponentialBackOff runs forever because Stop is never set. The CLA is signed and tests pass. Would appreciate a review when you have a moment. Thanks!