planner: fix column count mismatch error when push down Agg to UnionExec.

[wshwsh12]

What problem does this PR solve?

Issue Number: close #26554

Problem Summary: SQL gets an internal error: UnionExec chunk column count mismatch.
When pushing down Aggregate for Union, unionExec will use the children schema as itself.
Then the optimization columnPruner maybe try to prune the column of Aggregate, which wrongly modified the schema of Union. Because they use the same column slice...

We need clone the schema when building UnionExec.

What is changed and how it works?

Proposal: xxx

What's Changed:
Clone the schema for UnionExec.

How it Works:

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

Fix unexpected internal error when pushdown aggregate to union

[ti-chi-bot]

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• lzmhhh123

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[time-and-fate]

The KeyInfo is also cloned if we use Schema.Clone().
I think it's not expected here.