vector: shorten explain output for vector constants

[EricZequan]

What problem does this PR solve?

Issue Number: ref #54245

Problem Summary:

What changed and how does it work?

It is very likely that user passes a vector with very high dimension (e.g. dimension of 1000). Previously we output all values when such SQL is explained, which makes the explain hard to read.

This PR shortens the output and truncates the vector to display only first several elements.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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.

None

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##             feature/vector-search/vector-data-type     #55406   +/-   ##
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[EricZequan]

/cc @breezewish PTAL

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[breezewish]

ctx is not used at all. From Vector's perspective you should remove it. Consider that you are a Vector developer. What is ctx? They are all not used, and you don't even know what are them. They are totally not related with Vector at all, and Vector is not relying on them. So they should not be there.

And also, consider about the redact. For a Vector, there is no need to redact part of the string. A vector constant is either fully redacted (becomes ? or <..>), or not redacted at all. So this is the chance for Vector to not care about redact as well.

The previous name StringForExplain could be a good one, or you could use other names, as long as it clearly, precisely describe the functionality, for example, StringTruncated.

[breezewish]

Close for #55934