clientv3/concurrency: preserve leadership state on failed Resign

[AkashKumar7902]

Refs #22123.

What changed

• Preserve an election's local leadership state when Resign returns a transaction error.
• Add an integration regression proving that a failed resignation can be retried successfully.

Why

Election.Resign previously cleared leaderKey and leaderSession even when the delete transaction returned an error. If the request never reached etcd, the server-side leader key remained while the client forgot how to remove it.

The existing create-revision comparison makes a retry safe: it cannot delete a newer incarnation of the same key. A successful transaction response still retires the local state, including when the comparison does not match.

Impact

Callers can retry Resign after an RPC error. Successful resignation behavior and the exported API are unchanged.

This is complementary to #21165 and distinct from #19598 and #21128. Those changes concern Campaign cleanup or waiting-session expiration rather than errors returned by Resign itself.

Backport candidate: release-3.7 and release-3.6.

Testing

• Confirmed the regression fails on upstream main because the failed call clears Election.Key().
• cd tests && go test ./integration/clientv3/concurrency -run '^TestElectionResignCanRetryAfterError$' -count=20
• cd tests && go test ./integration/clientv3/concurrency -count=1
• cd tests && go test -race ./integration/clientv3/concurrency -run '^TestElectionResignCanRetryAfterError$' -count=1
• cd client/v3 && go test ./concurrency -count=1
• cd client/v3 && go vet ./concurrency
• make verify-lint
• git diff --check

The aggregate make verify cannot complete faithfully on this Darwin/arm64 host: the local BOM generator excludes the amd64-only Antithesis SDK and therefore differs from the checked-in Linux BOM. All non-BOM verify targets were run separately.

[kubernetes-prow]

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[kubernetes-prow]

Hi @AkashKumar7902. Thanks for your PR.

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