planner: fix wrong result when pushing Agg down through Union in MPP plans (#46310)

[ti-chi-bot]

This is an automated cherry-pick of #46310

What problem does this PR solve?

Issue Number: close #45850

Problem Summary:

What is changed and 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

Please refer to Release Notes Language Style Guide to write a quality release note.

planner: fix wrong result when pushing Agg down through Union in MPP plans

[ti-chi-bot]

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[ti-chi-bot]

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