perf: return Utf8View from Spark hex with inline short path

[shinzoxD]

Which issue does this PR close?

• Closes [datafusion-spark] Optimize hex function #15986

Rationale for this change

Review of #15947 asked Spark hex to return Utf8View instead of Utf8, with a fast path for encodings that fit in the StringView inline prefix (12 bytes). The lookup table (#21836) and criterion benches (#19738) already landed. This finishes the remaining item on #15986.

Short hex results are common (small integers, short strings/binaries). Building a Utf8 StringArray for those rows pays for offset buffers and a data buffer even when Arrow can store the digits in the view itself. Returning Utf8View avoids that for encodings of at most 12 bytes and still stores longer encodings out-of-line.

What changes are included in this PR?

• SparkHex::return_type is now Utf8View, including dictionary values (Dictionary(K, Utf8View)).
• Encodings of at most 12 hex digits are inlined via make_view and never touch the StringView data buffer.
• Longer encodings are written once into that buffer and referenced by offset.
• Input null buffers are still reused (ptr_eq on the inner validity buffer).
• BinaryView inputs are handled (the signature already accepted them).
• Criterion bench adds hex_utf8_inline for the short-string path. I am not posting machine-local timings here; please run cargo bench -p datafusion-spark --bench hex against main if you want a comparison.

Are these changes tested?

Yes.

• Unit tests in datafusion/spark/src/function/math/hex.rs cover Utf8View / LargeUtf8 / BinaryView inputs, null reuse, the 12- vs 13-digit inline boundary, and return_type.
• SQL logic tests in datafusion/sqllogictest/test_files/spark/math/hex.slt assert values plus arrow_typeof(hex(...)) = Utf8View (and Dictionary(Int32, Utf8View) for dictionary input).

Are there any user-facing changes?

Yes. hex now returns Utf8View instead of Utf8. Encoded values are unchanged. Callers that match on the physical type (for example ScalarValue::Utf8) need to accept Utf8View.

This is a public return-type change. I cannot add labels on this repo; please apply the api change label if that is still the right category.

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[Jefffrey]

for anyone looking at this PR please see my comment here:

• fix: roll back DiskManager usage when spill write I/O fails #24407 (comment)