planner: fix the issue that UnionScan returns wrong results in dynamic mode (#26755)

[ti-srebot]

cherry-pick #26755 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26843

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/26843:release-5.0-2745baad11c8

---

What problem does this PR solve?

Issue Number: close #26719

Problem Summary: planner: fix the issue that UnionScan returns wrong results in dynamic mode

What is changed and how it works?

planner: fix the issue that UnionScan returns wrong results in dynamic mode

Please see #26719 for more details.

Check List

Tests

• Unit test

Release note

planner: fix the issue that UnionScan returns wrong results in dynamic mode

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@qw4990 you're already a collaborator in bot's repo.

[qw4990]

Close since it's duplicated with #26876