planner: fix index merge reader may lost enforced prop sort when table plan is pure (#49844)

[ti-chi-bot]

This is an automated cherry-pick of #49844

What problem does this PR solve?

Issue Number: close https://github.com/pingcap/tidb/issues/49605

Problem Summary:

What changed and how does it work?

Check List

Tests

• [x] 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 may lost enforce prop sort when table plan is pure

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (release-7.1@4d53073). Click here to learn what that means. The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.1     #49889   +/-   ##
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  Coverage               ?   73.4300%           
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  Lines                  ?     378886           
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  Hits                   ?     278216           
  Misses                 ?      82910           
  Partials               ?      17760           

[hawkingrei]

/retest

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• ~~OWNERS~~ [AilinKid]

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-02-26 08:17:57.127388242 +0000 UTC m=+863565.875011354: :ballot_box_with_check: agreed by fixdb.
• 2024-02-27 06:52:26.701610729 +0000 UTC m=+944835.449233840: :ballot_box_with_check: agreed by AilinKid.

[winoros]

/retest