server: avoid health API panic before etcd starts

[rleungx]

What problem does this PR solve?

Issue Number: Close #10978

What is changed and how does it work?

The PD health API can be called while PD is serving HTTP requests but the etcd client is not ready yet. In that state, member lookup could dereference a nil etcd client and panic.

This change makes member lookup return ErrEtcdNotStarted when the etcd client is nil, and makes the health API return HTTP 503 for unavailable startup state instead of HTTP 500 or a process-level panic. It also documents the 503 response in the API annotation.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has HTTP APIs changed (Don't forget to add the declarative for the new API)

Release note

Fix a PD health API panic that could happen during cluster startup or restart.

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Review details

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Configuration used: defaults

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Run ID: 969dc081-9691-40ac-9cfd-b53636e3f6fd

📥 Commits

Reviewing files that changed from the base of the PR and between 14c2449 and d9e9f03.

📒 Files selected for processing (3)

• server/api/health.go
• server/api/health_test.go
• server/cluster/cluster.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

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[rleungx]

/test pull-error-log-review

[rleungx]

/test pull-unit-test-next-gen-2
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/test pull-unit-test-next-gen-2
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[rleungx]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.27%. Comparing base (8abdf30) to head (d9e9f03).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10982   +/-   ##
=======================================
  Coverage   79.26%   79.27%           
=======================================
  Files         541      541           
  Lines       75674    75688   +14     
=======================================
+ Hits        59985    59999   +14     
+ Misses      11467    11466    -1     
- Partials     4222     4223    +1     

Flag	Coverage Δ
unittests	79.27% <100.00%> (+<0.01%)	⬆️

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[rleungx]

/test pull-unit-test-next-gen-2

[rleungx]

/test pull-unit-test-next-gen-3

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bufferflies, lhy1024

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• OWNERS [bufferflies,lhy1024]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-07-06 11:34:14.567262265 +0000 UTC m=+22240.603357311: ☑️ agreed by bufferflies.
• 2026-07-06 14:02:54.189280338 +0000 UTC m=+31160.225375394: ☑️ agreed by lhy1024.