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

[ti-srebot]

cherry-pick #30981 to release-5.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/30992

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

git push git@github.com:ti-srebot/tidb.git pr/30992:release-5.0-404895c04a51

---

What problem does this PR solve?

Issue Number: close #30971

Problem Summary:

What is changed and how it works?

Return the correct fields count for the prepare statement...
The old code only consider the 'select' statement and ignore all the other cases.
And 'select ... union select ...' is a counter-example trigger the bug.

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.

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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

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