planner: fix index merge union case couldn't sink limit into index merge reader (#48590)

[ti-chi-bot]

This is an automated cherry-pick of #48590

What problem does this PR solve?

Issue Number: close #48588

Problem Summary:

What is changed and how it works?

detail ref the comment #48588 (comment) in the issue above

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.

planner: fix index merge union case couldn't sink limit into index merge reader

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

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[codecov]

Codecov Report

Attention: Patch coverage is 73.68421% with 5 lines in your changes are missing coverage. Please review.

Please upload report for BASE (release-7.5@932994b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #48632   +/-   ##
================================================
  Coverage               ?   74.9993%           
================================================
  Files                  ?       1412           
  Lines                  ?     437120           
  Branches               ?          0           
================================================
  Hits                   ?     327837           
  Misses                 ?      89970           
  Partials               ?      19313           

Flag	Coverage Δ
unit	74.9993% <73.6842%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	54.8388% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, qw4990

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Needs approval from an approver in each of these files:

• OWNERS [AilinKid,qw4990]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-21 06:43:30.246973963 +0000 UTC m=+2153964.004109535: ☑️ agreed by qw4990.
• 2024-05-21 06:44:09.020734747 +0000 UTC m=+2154002.777870321: ☑️ agreed by AilinKid.

[qw4990]

/retest