tests: check completed leader handoff in TSO startup test

[rleungx]

What problem does this PR solve?

Issue Number: ref #8836

TestRaftClusterStartTSOJob can fail on a leader-transfer timeout even after another peer has become leader. In the failing CI run, the campaign loop superseded the test's transfer to pd3 with a transfer to pd1. The original request timed out while waiting for its specific target, so the test stopped before checking TSO allocator shutdown.

What is changed and how does it work?

Request resignation from the captured original leader and wait for all servers
to agree on a different leader before checking that the old TSO allocator is
uninitialized. Include the resignation error in the leader-change assertion's
failure message.

The four startup scenarios and existing allocator assertions are preserved. The test still fails if leadership does not change.

Check List

Tests

• Unit test: TestRaftClusterStartTSOJob, 3 runs each with -race -tags deadlock and -race -tags deadlock,nextgen.
• Baseline comparison: 3 runs with -race -tags deadlock,nextgen.
• Controlled regression: after a real leader transfer, inject the timeout response; the original test fails and the updated test passes.
• Negative check: acknowledge resignation without changing leadership; the updated test fails at WaitLeaderChange.
• make check with golangci-lint v2.6.0.

The CI timing did not reproduce in the unmodified baseline runs; the controlled regression validates the observed completed-transfer/error-response outcome. Fault injection is confined to disposable validation worktrees.

Release note

None.

Summary by CodeRabbit

• Tests
  • Improved cluster leadership-change test reliability by explicitly waiting for leadership transition completion.
  • Leadership resignation errors are now included in test assertions.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: ae4c5d42-f7d1-4fe6-9830-8369bf1dda37

📥 Commits

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📒 Files selected for processing (1)

• tests/server/cluster/cluster_test.go

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📝 Walkthrough

Walkthrough

TestRaftClusterStartTSOJob now explicitly resigns the original leader and waits for the completed leadership change. The assertion also records any resignation error.

Changes

Leadership change testing

Layer / File(s)	Summary
Synchronize leader resignation tests/server/cluster/cluster_test.go	The test replaces the generic resignation and leader wait with leaderServer.ResignLeader() and WaitLeaderChange(name). The assertion includes any resignation error.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: jmpotato

Merge Risk: ⚪ Minimal · up to 79b33

This test-only synchronization change has no identified merge-blocking behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the test change and the completed leader handoff it verifies.
Description check	✅ Passed	The description includes the issue reference, problem, implementation details, validation results, checklist information, and release note. It matches the required template and provides sufficient con…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[rleungx]

/retest

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.74%. Comparing base (ef560c8) to head (79b3350).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11255   +/-   ##
=======================================
  Coverage   79.73%   79.74%           
=======================================
  Files         546      546           
  Lines       79262    79262           
=======================================
+ Hits        63203    63206    +3     
+ Misses      11686    11675   -11     
- Partials     4373     4381    +8     

Flag	Coverage Δ
unittests	79.74% <ø> (+<0.01%)	⬆️

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