planner: Fix vector not truncated after CBO

[breezewish]

What problem does this PR solve?

Issue Number: Close #58837

Problem Summary:

What changed and how does it work?

After CBO is introduced, vector is not truncated in explain. This is what it looks like now without this PR:

Upstream PR: https://github.com/tidbcloud/tidb-cse/pull/1416

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.

None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.9975%. Comparing base (2a3542c) to head (3e2185f).
Report is 14 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58809        +/-   ##
================================================
+ Coverage   73.0827%   73.9975%   +0.9148%     
================================================
  Files          1676       1677         +1     
  Lines        463747     467987      +4240     
================================================
+ Hits         338919     346299      +7380     
+ Misses       103979     100965      -3014     
+ Partials      20849      20723       -126     

Flag	Coverage Δ
integration	43.5976% <0.0000%> (?)
unit	72.8047% <100.0000%> (+0.5321%)	⬆️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.7389% <ø> (+0.0059%)	⬆️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: winoros, XuHuaiyu

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Needs approval from an approver in each of these files:

• pkg/expression/OWNERS [XuHuaiyu]
• pkg/planner/OWNERS [winoros]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-09 04:40:52.156548239 +0000 UTC m=+414995.445379944: ☑️ agreed by XuHuaiyu.
• 2025-01-09 18:13:18.725620856 +0000 UTC m=+463742.014452559: ☑️ agreed by winoros.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #58844.