external store: use separate ctx for GCS clients (#60402)

[ti-chi-bot]

This is an automated cherry-pick of #60402

What problem does this PR solve?

Issue Number: close #60155

Problem Summary:

What changed and how does it work?

see original issue for more information

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]

@lance6716 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.

[fzzf678]

/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fzzf678, lance6716

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

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Needs approval from an approver in each of these files:

• br/pkg/storage/OWNERS [lance6716]

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:

• 2025-07-02 01:48:05.628630649 +0000 UTC m=+1445938.351809626: ☑️ agreed by fzzf678.
• 2025-07-02 02:22:27.487662926 +0000 UTC m=+1448000.210841908: ☑️ agreed by lance6716.

[lance6716]

/hold

the new test case failed

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 21 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-8.1@61d7f3c). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #61919   +/-   ##
================================================
  Coverage               ?   71.2626%           
================================================
  Files                  ?       1468           
  Lines                  ?     424284           
  Branches               ?          0           
================================================
  Hits                   ?     302356           
  Misses                 ?     101415           
  Partials               ?      20513           

Flag	Coverage Δ
unit	71.2626% <50.0000%> (?)

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

Components	Coverage Δ
dumpling	52.9656% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.4454% <0.0000%> (?)

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[fzzf678]

/unhold

[fzzf678]

/retest

[fzzf678]

/retest

[fzzf678]

/retest