br: fix region scan checker (#64530)

[ti-chi-bot]

This is an automated cherry-pick of #64530

What problem does this PR solve?

Issue Number: close #64514

Problem Summary:
the store id of region leader scanned may be 0.

What changed and how does it work?

add region scan checker to check the leader store id after each region scan API.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
2. For pull requests to TiDB-x branches, it must have no failed tests.
3. AFTER it has lgtm and approved labels, please wait for the cherry-pick merging approval from triage owners.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[codecov]

Codecov Report

❌ Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-nextgen-20251011@38a7550). Learn more about missing BASE report.

Additional details and impacted files

@@                      Coverage Diff                      @@
##             release-nextgen-20251011     #65796   +/-   ##
=============================================================
  Coverage                            ?   71.8610%           
=============================================================
  Files                               ?       1828           
  Lines                               ?     491042           
  Branches                            ?          0           
=============================================================
  Hits                                ?     352868           
  Misses                              ?     114970           
  Partials                            ?      23204           

Flag	Coverage Δ
unit	71.8610% <89.7435%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.8700% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	46.5037% <0.0000%> (?)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ti-chi-bot]

@wiaogoelgiaiux: adding LGTM is restricted to approvers and reviewers in OWNERS files.

Details

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RidRisR, wiaogoelgiaiux, YuJuncen

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

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• br/OWNERS [YuJuncen]

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:

• 2026-01-26 03:01:16.437921152 +0000 UTC m=+981304.051877998: ☑️ agreed by YuJuncen.
• 2026-01-26 06:25:32.288642446 +0000 UTC m=+993559.902599301: ☑️ agreed by RidRisR.