external: fix decoding the last key in split keys (#59613)

[ti-chi-bot]

This is an automated cherry-pick of #59613

What problem does this PR solve?

Issue Number: close #59725

Problem Summary:

When the kv size or kv count of unique key is large, we will call GetRegionSplitKeys, which did not properly consider the case of key.Next(). Then it fails due to the decoding.

What changed and how does it work?

Introduce the method tryDecodeEndKey to decode the last key

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.

Fix a bug that fails to add unique key using global sort in large data

[ti-chi-bot]

@CbcWestwolf 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]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: CbcWestwolf
Once this PR has been reviewed and has the lgtm label, please assign benjamin2037 for approval. For more information see the Code Review Process.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-25 08:27:21.435900671 +0000 UTC m=+344389.389058933: ☑️ agreed by CbcWestwolf.

[CbcWestwolf]

/unhold

[CbcWestwolf]

/retest

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 11 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-8.1@3dce1ce). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #59747   +/-   ##
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  Coverage               ?   71.2087%           
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  Files                  ?       1468           
  Lines                  ?     424025           
  Branches               ?          0           
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  Hits                   ?     301943           
  Misses                 ?     101539           
  Partials               ?      20543           

Flag	Coverage Δ
unit	71.2087% <66.6666%> (?)

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	41.4273% <0.0000%> (?)

[CbcWestwolf]

/retest