perf: remove per-row String allocations from the Spark url functions

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Three allocation problems in the url module, all on the per-row path.

url_encode built a String for every row:

fn encode(value: &str) -> Result<String> {
    Ok(byte_serialize(value.as_bytes()).collect::<String>())
}

url_decode ended with .map(|parsed| parsed.into_owned()). Both
replace_plus and decode_utf8 return a Cow that borrows when there is
nothing to rewrite, so into_owned allocated a String for every row even when
the value contained no percent-escapes and no + at all.

parse_url built the all-null key array for the two-argument form by
calling append_null() once per row against a builder with no capacity.

What changes are included in this PR?

url_encode.rs:

• The three type branches build a pre-sized builder and share an encode_all!
  loop that clears and refills one scratch String per batch.
• UrlEncode::encode is removed; it returned a Result whose error arm was
  never constructed and now has no callers.

url_decode.rs:

• decode returns Cow<'_, str> instead of String. When replace_plus
  borrows, the decode borrows straight from the input; when it has already
  allocated (the input contained +), owning the decoded form costs nothing
  beyond what was already spent.
• The array paths append the Cow to a pre-sized builder rather than
  collecting Result<StringArray> from a per-row String.
• API change: spark_handled_url_decode's second parameter changes from
  impl Fn(Result<Option<String>>) -> Result<Option<String>> to a new
  OnDecodeError enum. The String in that signature is what forced the
  allocation. Its only caller is try_url_decode, whose closure was
  Err(_) => Ok(None) — exactly OnDecodeError::Null.

parse_url.rs:

• The append_null()-per-row loop becomes new_null_array(&DataType::Utf8, len).

No behaviour change in any of the three.

Are these changes tested?

Existing coverage pins the behaviour: spark/url/url_encode.slt,
url_decode.slt, try_url_decode.slt, and parse_url.slt assert concrete
results including reserved characters, + handling, malformed percent-encoding
(which must error for url_decode and yield NULL for try_url_decode), and the
two- versus three-argument parse_url forms. All 5 spark/url sqllogictest
files pass, along with the 258 datafusion-spark unit tests.

The try_url_decode unit test is what pins the OnDecodeError::Null path,
since it drives a malformed input through the refactored signature.

Benchmarks are added separately in #23882 so the baselines can be measured on
main before this lands.

Benchmarks

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

url_encode:

Benchmark	1024	8192
url_encode/utf8	−57.1%	−48.2%
url_encode/largeutf8	−56.1%	−49.7%
url_encode/utf8view	−56.0%	−46.5%

url_decode, split by whether the input actually needs unescaping:

Benchmark	1024	8192
url_decode/plain_utf8	−26.4%	−28.8%
url_decode/plain_utf8view	−29.0%	−27.2%
url_decode/escaped_utf8	−5.4%	−3.6%
url_decode/escaped_largeutf8	−4.7%	−3.2%

The plain rows are the ones that benefit: with nothing to unescape the decoded
value borrows its input. The escaped rows still have to allocate, so they gain
only the pre-sized builder and the removed intermediate — a few percent, as
expected.

parse_url has no benchmark; its change removes an append_null() loop that
runs once per batch rather than affecting a measured per-row path.

Are there any user-facing changes?

No change to SQL behaviour — all three functions return identical results.

spark_handled_url_decode is public Rust API and its signature changes as
described above; OnDecodeError is new public API on datafusion-spark.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.92308% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (f1ab86d) to head (3b3a748).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/url/url_decode.rs	77.27%	0 Missing and 5 partials ⚠️
datafusion/spark/src/function/url/url_encode.rs	72.72%	0 Missing and 3 partials ⚠️
datafusion/spark/src/function/url/parse_url.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23884      +/-   ##
==========================================
- Coverage   80.65%   80.65%   -0.01%     
==========================================
  Files        1091     1092       +1     
  Lines      371031   371089      +58     
  Branches   371031   371089      +58     
==========================================
+ Hits       299256   299293      +37     
- Misses      53935    53946      +11     
- Partials    17840    17850      +10     

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