expression: make built-in function ord() backward compatible

[tangenta]

What problem does this PR solve?

Related to #30156.

Problem Summary:

select ord(_latin1 '你');

should not return an encoding error.

Previous ord() implementation did not consider the backward compatibility. The test TestStringBuiltin was skipped due to the test framework refactoring. Now we add it back.

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

None

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[ti-chi-bot]

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[tangenta]

cc @unconsolable

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@hawkingrei: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

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[hawkingrei]

/run-check_dev_2

[tangenta]

It will be fixed in #30288.