types: fix wrong str_to_date() microseconds with leading zeros are not converted correctly (#30122)

[ti-srebot]

cherry-pick #30122 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/30219

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git push git@github.com:ti-srebot/tidb.git pr/30219:release-4.0-352811dfc4ee

---

Signed-off-by: wjhuang2016 huangwenjun1997@gmail.com

What problem does this PR solve?

Issue Number: close #30078

str_date doesn't parse microseconds correctly.

What is changed and how it works?

It's very easy. Not sure why it was wrong before.

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

None

[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

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[xhebox]

close for DMR version