br: Use int for vlen to prevent overflow (#63662)

[ti-chi-bot]

This is an automated cherry-pick of #63662

What problem does this PR solve?

Issue Number: close #63663

Problem Summary:

What changed and how does it work?

Change vlen to int before use it as a length flag to avoid parse overflow.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-8.5@1c16548). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #63676   +/-   ##
================================================
  Coverage               ?   57.8206%           
================================================
  Files                  ?       1783           
  Lines                  ?     632962           
  Branches               ?          0           
================================================
  Hits                   ?     365983           
  Misses                 ?     242260           
  Partials               ?      24719           

Flag	Coverage Δ
integration	38.2741% <40.0000%> (?)
unit	72.8101% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	63.4542% <0.0000%> (?)

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[RidRisR]

/run-mysql-test

[RidRisR]

/run-mysql-test

[ti-chi-bot]

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[BornChanger]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger, Leavrth, RidRisR

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Needs approval from an approver in each of these files:

• br/OWNERS [BornChanger,Leavrth]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-23 07:03:07.23351078 +0000 UTC m=+340197.304004463: ☑️ agreed by BornChanger.
• 2025-09-23 08:06:59.081646017 +0000 UTC m=+344029.152139733: ☑️ agreed by Leavrth.