perf(substr_index): speed up scalar and Utf8View

[kumarUjjawal]

Which issue does this PR close?

• Part of feat: Update string UDF's to have return type == input type where appropriate #20585.

Rationale for this change

substr_index is part of the Utf8View epic. This function was still missing the Utf8View optimization path, and it also did extra work when delimiter and count were constant scalars.

This PR adds the same kind of optimization used in similar string work and includes benchmark coverage.

What changes are included in this PR?

• Return the input string type directly, including Utf8View.
• Add a zero-copy Utf8View path for substr_index.
• Add a scalar fast path when delimiter and count are constant.
• Keep the hot path specialized after checking a cleaner shared-helper version and benchmarking it.
• Add SQL coverage for Utf8View type and value results.
• Add unit tests for the scalar fast path, sliced Utf8View arrays, and the unchanged original-view case.
• Extend the benchmark to cover Utf8, Utf8View, array arguments, scalar delimiter/count, and positive and negative counts.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[kumarUjjawal]

cc @neilconway since you are working on this area.

[neilconway]

@kumarUjjawal Thanks for working on this! Can you post some benchmark data, please?

[neilconway]

Otherwise lgtm!

[neilconway]

Here and below, this over-allocates the buffer capacity for sliced arrays.

[kumarUjjawal]

@kumarUjjawal Thanks for working on this! Can you post some benchmark data, please?

Case	Before	After	Result
Utf8 scalar, single delimiter, count = 1	20.18 us	11.29 us	1.79x faster
Utf8 scalar, long delimiter, count = 1	39.82 us	13.69 us	2.91x faster
Utf8View array, single delimiter	26.64 us	11.77 us	2.26x faster
Utf8View scalar, long delimiter, count = 1	40.59 us	12.88 us	3.15x faster
Utf8View array, long delimiter	52.47 us	46.28 us	1.13x faster

[kumarUjjawal]

Thanks @neilconway

[alamb]

Thanks @kumarUjjawal and @neilconway