server, member, election: document and test the election client pinning (#11110)

[ti-chi-bot]

This is an automated cherry-pick of #11110

What problem does this PR solve?

Issue Number: ref #11106, ref #10671, ref #10746, ref #7780

#9986 made the etcd client health checker optional and created the election client with it disabled. The election client is built from the member's own advertise client URLs, and only the health checker rewrites that endpoint list, so leaving it off confines leader lease renewals to the local etcd server.

The constraint is not recorded anywhere in the tree and no test asserts it. This PR supplies the missing documentation and tests. It changes no behaviour.

What is changed and how does it work?

Record why the election client's health checker must stay disabled: at the call
site, and in newClient, where AutoSyncInterval must remain unset. Document on
GetEtcdLeader what may and may not depend on it, listing every current caller.

Add three integration tests. The first asserts that the election client's
endpoints stay equal to the local advertise URLs while the health checked
server client discovers its peers. The other two assert that a member ends its
term when its renewals fail locally, and when its lease is revoked, in both
cases with the colocation check blinded so that it cannot be what fires.

Two failpoints support them: staleEtcdLeaderView on Member.GetEtcdLeader,
matched by member name, and keepAliveFailed in the lease keepalive worker,
matched as "<purpose>@<name>" so that enabling it affects a single member of an
in-process cluster.

No behaviour change.

Check List

Tests

• Integration test

Release note

None.

Summary by CodeRabbit

• Bug Fixes

  • Leadership now steps down promptly when its lease is revoked or renewal fails, including when status information is temporarily stale.
  • Election connections remain tied to the local member, improving leadership-lease validation and preventing misleading continued leadership.

• Tests

  • Added coverage for lease-loss handling, failed renewals, stale status views, and local election connection behavior.

• Documentation

  • Clarified expected election-client and leadership behavior during connectivity or progress changes.

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[ti-chi-bot]

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@ti-chi-bot: 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-build-release-8.5	352d0bd	link	true	/test pull-build-release-8.5

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