types: fix a bug in casting str2str when union (#37242)

[ti-srebot]

cherry-pick #37242 to release-6.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/37361

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

git push git@github.com:ti-srebot/tidb.git pr/37361:release-6.0-25dda978ff9f

---

What problem does this PR solve?

Issue Number: close #31678

Problem Summary:

When building the plan of union, the flen of the projection target FieldType is mis-computed.

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.

Fix a wrong casting in building union plan

[ti-chi-bot]

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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

[xhebox]

closed for DMR version