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GH-36173: [C++] Add lone high and low code-point test case for UTF8StringToUTF16 (#36383)



### Rationale for this change

This is a followup PR to #36167 that addresses feedback left after the PR was merged.

### What changes are included in this PR?

1. Added a test point verifying `UTF8StringToUTF16` returns an `Invalid` status if given a UTF-8 encoded string that contains a lone high or low code point.
2. Removed `ARROW_EXPORT` from definitions of `UTF8StringToUTF16` and `UTF16StringToUTF18`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* Closes: #36173

Lead-authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Co-authored-by: sgilmore10 <74676073+sgilmore10@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>