ddl: remove unnecessary `PrefixNext()` during `iterateSnapshotKeys()`

[tangenta]

What problem does this PR solve?

Issue Number: close #61026

Problem Summary:

See #61026

What changed and how does it work?

Remove unnecessary PrefixNext() during iterateSnapshotKeys().

Check List

Tests

• [ ] Unit test
• [ ] Integration test
• [x] 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

[tiprow[bot]]

Hi @tangenta. Thanks for your PR.

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 78.3220%. Comparing base (c157641) to head (1d0878c). :warning: Report is 862 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61027        +/-   ##
================================================
+ Coverage   73.1297%   78.3220%   +5.1922%     
================================================
  Files          1723       1897       +174     
  Lines        476955     576248     +99293     
================================================
+ Hits         348796     451329    +102533     
+ Misses       106742     101422      -5320     
- Partials      21417      23497      +2080     

Flag	Coverage Δ
integration	48.8316% <100.0000%> (?)
unit	76.9527% <100.0000%> (+4.6057%)	:arrow_up:

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Components	Coverage Δ
dumpling	52.9532% <ø> (+0.1728%)	:arrow_up:
parser	∅ <ø> (∅)
br	61.9001% <ø> (+14.4009%)	:arrow_up:

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[tangenta]

/retest

[tiprow[bot]]

@tangenta: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wjhuang2016

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-06-18 12:05:59.150435366 +0000 UTC m=+273411.873614343: :ballot_box_with_check: agreed by wjhuang2016.