server/api: allow follower local region reads (#10682)

[ti-chi-bot]

This is an automated cherry-pick of #10682

What problem does this PR solve?

Issue Number: Close #10681

What is changed and how does it work?

Allow safe read-only region HTTP APIs to use the follower's local synced region cache when `PD-Allow-Follower-Handle: true` is set and the region syncer is running.

Keep normal redirect behavior unchanged for requests without the opt-in header, and keep follower-local handling limited to read-only region inspection paths.

Check List

Tests

• Unit test

Code changes

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

Release note

PD followers can now serve read-only region HTTP APIs from their local synced region cache when explicitly requested.

Summary by CodeRabbit

• New Features

  • Follower-synced nodes can directly serve many region read endpoints (lookups, lists/counts, topology and read stats) when a specific GET header is present, improving read availability when the leader is not serving. Region-read routes now use the follower-capable middleware.

• Bug Fixes / Behavior

  • Certain region check endpoints still respond as unbootstrapped even with the allow header.

• Tests

  • Added test verifying follower-served region responses and unchanged behavior for some endpoints.

• Documentation

  • Clarified comment on a helper used for follower-local access.

[ti-chi-bot]

@okJiang This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[okJiang]

Please pull the latest base branch and resolve the cherry-pick conflicts

[okJiang]

/retest

[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.85%. Comparing base (d54de44) to head (21b8625).
⚠️ Report is 1 commits behind head on release-8.5-20251204-v8.5.4.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           release-8.5-20251204-v8.5.4   #10694      +/-   ##
===============================================================
+ Coverage                        77.78%   77.85%   +0.06%     
===============================================================
  Files                              469      469              
  Lines                            63087    63108      +21     
===============================================================
+ Hits                             49075    49132      +57     
+ Misses                           10415    10379      -36     
  Partials                          3597     3597              

Flag	Coverage Δ
unittests	77.85% <95.34%> (+0.06%)	⬆️

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[okJiang]

/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

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Details

Needs approval from an approver in each of these files:

• OWNERS [bufferflies]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-22 08:10:39.444987989 +0000 UTC m=+110970.085844792: ☑️ agreed by okJiang.
• 2026-05-22 08:32:24.798403461 +0000 UTC m=+1085.305115120: ☑️ agreed by bufferflies.

[okJiang]

/unhold