planner: fix wrong plan caused by shallow copy schema columns (#27798)

[ti-srebot]

cherry-pick #27798 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/27838

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---

What problem does this PR solve?

Issue Number: close #27797 close #26554

Problem Summary:
In agg push down, we make the schema of the Union and the first child of the Union(a DataSource in the case) share the same array.
Then in the column pruning, when we are pruning columns of the child of the Union, the schema of Union is also changed because they share the same slice.

What is changed and how it works?

Deep copy schema columns when push agg across Union.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qw4990
• wjhuang2016

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[ti-srebot]

/run-all-tests

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[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 854de04

[zhouqiang-cl]

@ti-chi-bot

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