test(leader-election): cover follower leader observation

[GatewayJ]

Type of Change

• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Refactor
• Other:

Related Issues

• Closes rustfs/backlog#1083

Summary of Changes

• Replace the pre-seeded single-follower regression test with three active electors sharing one lock backend.
• Verify that both followers publish Following("node-1"), expose the leader through current_leader(), and never report themselves as leaders.
• Verify that each follower receives exactly one on_new_leader("node-1") callback while preserving the stopped callback contract.
• Keep failure injection state isolated per fake lock participant while sharing only distributed lock state.

The production behavior was already corrected by #194. This change completes the explicit three-instance acceptance coverage for rustfs/backlog#1083 without changing production code or public APIs.

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit (fmt-check + clippy + test + console-lint + console-fmt-check)
• Added/updated necessary tests
• Documentation updated (N/A: test-only change)
• CHANGELOG.md updated under [Unreleased] (N/A: test-only change)
• CI/CD passed (pending)

Impact

• Breaking change (CRD/API compatibility)
• Requires doc/config/deployment update
• Other impact: regression coverage only; no production behavior or public API changes

Verification

make pre-commit
cargo test -p kube-leader-election test_spawn_reports_remote_leader_to_all_followers -- --exact

The focused three-instance test was also run 100 consecutive times successfully.

Additional Notes

The test starts node-1 first and waits for Leading before starting two followers. Followers are stopped and joined before the leader to avoid release or takeover races.

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Thank you for your contribution! Please ensure the community standards (CODE_OF_CONDUCT.md) are followed and the CLA is signed if this is your first contribution.

[GatewayJ]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: 4a62f6b84f

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