perf: optimize spark soundex function

[kazantsev-maksim]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

• Replaced dynamic string building with a fixed-size stack array [b'0'; 4] initialized with zeros. The code is filled in place and converted instantly using unsafe { std::str::from_utf8_unchecked(&buf).to_string() }, resulting in zero heap allocations for code formatting.
• Added an ASCII Fast-Path (s.is_ascii()). When inputs consist entirely of ASCII characters, the function operates directly on raw UTF-8 byte slices (&[u8]) with optimized byte classification (classify_byte), falling back to the Unicode char path only for non-ASCII text.
• Replaced .collect() with GenericStringBuilder::::with_capacity(rows, rows * 4) and StringBuilder::with_capacity for Utf8View. Since every Soundex result is guaranteed to be exactly 4 bytes, memory for the entire output column is pre-allocated in a single block, eliminating column-level heap re-allocations entirely.

Are these changes tested?

Existing tests.

Benchmark (criterion):

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.28571% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (88365dd) to head (0cb31d5).

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/string/soundex.rs	84.28%	10 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23909      +/-   ##
==========================================
- Coverage   80.65%   80.65%   -0.01%     
==========================================
  Files        1093     1093              
  Lines      371619   371671      +52     
  Branches   371619   371671      +52     
==========================================
+ Hits       299730   299756      +26     
- Misses      53993    54017      +24     
- Partials    17896    17898       +2     

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[andygrove]

There is another soundex optimization PR open. #23880