br/stream: allow pitr to create oversized indices (#58433)

[ti-chi-bot]

This is an automated cherry-pick of #58433

What problem does this PR solve?

Issue Number: close #58430

Problem Summary:
During restoring, the internal config of BR wasn't updated.
It may reject all indices longer than the default config (3072) though the target cluster modified this config.

What changed and how does it work?

This updates the TiDB config of the BR internal TiDB instance during restoring incremental data.

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.

Fixed a bug that caused cannot restore index created during log backup if it is longer than `3072`.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-7.5@738a65b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #58529   +/-   ##
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  Coverage               ?   72.1476%           
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  Files                  ?       1417           
  Lines                  ?     413836           
  Branches               ?          0           
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  Hits                   ?     298573           
  Misses                 ?      95244           
  Partials               ?      20019           

Flag	Coverage Δ
unit	72.1476% <ø> (?)

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

Components	Coverage Δ
dumpling	52.9400% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.5019% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger, YuJuncen

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Needs approval from an approver in each of these files:

• br/OWNERS [BornChanger,YuJuncen]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-25 08:04:42.827845468 +0000 UTC m=+343030.781003732: ☑️ agreed by BornChanger.
• 2025-02-27 02:34:23.348974066 +0000 UTC m=+496011.302132332: ☑️ agreed by YuJuncen.

[BornChanger]

/retest