executor: fix the returned field count of the prepare statement #30981

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #30971

Problem Summary:

Cherry-pick to a hotfix branch

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

Fix a bug that the `mysql_stmt_field_count` returned to mysql client is incorrect in prepare protocal when handling union statement.

[ti-chi-bot]

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[ti-chi-bot]

@tiancaiamao: 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.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[zhouqiang-cl]

/run-build-arm64 comment=true