perf: avoid per-row String allocation in Spark bin and char

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Two Spark functions allocated a String per row purely to render a small,
bounded amount of text:

• bin called format!("{value:b}") for every row.
• char called ch.to_string() for every row — a heap allocation for a single
  character.

In both cases the output has a known upper bound (64 binary digits for an i64,
4 bytes for a UTF-8 character), so the rendering fits in a stack buffer and the
result can be appended straight to a pre-sized builder.

What changes are included in this PR?

math/bin.rs:

• spark_bin now writes digits right-aligned into a caller-supplied [u8; 64]
  and returns a &str borrowed from it, instead of returning an owned String.
• The collect::<StringArray>() becomes an explicit loop over a
  StringBuilder::with_capacity, sized at 8 digits per row.
• Negative values still render as their two's-complement bit pattern, matching
  {:b}. The digit loop is a loop, not a while, so zero renders as "0"
  rather than the empty string.

string/char.rs:

• ch.to_string() becomes ch.encode_utf8(&mut encoded) against a [u8; 4]
  hoisted out of the loop.

Output is unchanged in both cases.

Are these changes tested?

Existing coverage pins the behaviour. spark/math/bin.slt asserts concrete
output for the cases the rewrite had to get right: zero, negative values,
i64::MIN (-9223372036854775808), i64::MAX, and -2147483648 /
-32768 widened from narrower integer types. spark/string/char.slt covers
the negative-input empty string, the null path, and characters on both sides of
the ASCII boundary (char(256) and above wrap via % 256).

All 119 spark/math and spark/string sqllogictest files pass, along with the
258 datafusion-spark unit tests.

The bin benchmark used below, datafusion/spark/benches/bin.rs, is added
separately in #23882 so the baseline can be measured on main before this
change lands. It covers 1024 and 8192 rows with 20% nulls over two value
distributions: small values that render to a handful of digits, and full-range
values that render to the maximum 64. char already had
datafusion/spark/benches/char.rs on main, so no benchmark change is needed
for it.

Benchmarks

Criterion, apache/main @ f1ab86dad as baseline. Median of the reported
change interval.

Benchmark	1024	8192
bin/small	−75.7%	−73.7%
bin/wide	−47.8%	−49.8%

char (1024 rows): −76.1%.

Are there any user-facing changes?

No. Both functions produce byte-identical output; this is purely an allocation
change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.65%. Comparing base (f1ab86d) to head (8c59488).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23881      +/-   ##
==========================================
- Coverage   80.65%   80.65%   -0.01%     
==========================================
  Files        1091     1091              
  Lines      371031   371043      +12     
  Branches   371031   371043      +12     
==========================================
- Hits       299256   299254       -2     
- Misses      53935    53941       +6     
- Partials    17840    17848       +8     

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[neilconway]

lgtm!