Optimize Spark hex null handling

[floze-the-genius]

Which issue does this PR close?

• Closes Further optimize Spark hex byte encoding: reuse input NullBuffer and special-case the no-nulls path #23674.

Rationale for this change

Spark hex encoding preserves input validity exactly, but the byte-array path rebuilt the null bitmap row by row and always iterated through Option values. The input arrays already expose both the original NullBuffer and an all-valid fast path.

What changes are included in this PR?

• pass the byte array accessor into hex_encode_bytes so the output can clone and reuse the input NullBuffer
• split nullable and no-null loops, avoiding per-row validity checks when the input has no null buffer
• preserve exact offset, overflow, casing, dictionary, and null semantics
• add a sliced-array regression that verifies semantic equality and bitmap pointer reuse
• add no-null output coverage and dedicated UTF-8/binary Criterion cases

Are these changes tested?

Yes.

• cargo test -p datafusion-spark function::math::hex --lib: 10 passed
• cargo fmt --all -- --check: passed
• cargo clippy -p datafusion-spark --all-targets --all-features --no-deps -- -D warnings: passed
• git diff --check: passed
• benchmark target compiled with --features core

The workspace dependency clippy invocation without --no-deps reaches an unrelated existing dead_code warning in datafusion/physical-expr/src/expressions/dynamic_filters/mod.rs:464; the changed crate is clean.

Benchmark

Command (same warmed build and profile for upstream main and this branch):

CARGO_BUILD_JOBS=2 \
CARGO_PROFILE_BENCH_LTO=false \
CARGO_PROFILE_BENCH_CODEGEN_UNITS=16 \
cargo bench -p datafusion-spark --features core --bench hex -- \
  'hex_(utf8|binary)_no_nulls'

Mean times from the immediate upstream-then-branch sequence:

case	upstream main	this PR	change
UTF-8, 1,024 rows	79.895 us	59.181 us	-25.9%
UTF-8, 4,096 rows	317.64 us	244.88 us	-22.9%
UTF-8, 8,192 rows	509.64 us	411.27 us	-19.3%
Binary, 1,024 rows	86.135 us	40.600 us	-52.9%
Binary, 4,096 rows	346.16 us	253.13 us	-26.9%
Binary, 8,192 rows	635.98 us	424.78 us	-33.2%

Are there any user-facing changes?

No API or behavior changes. This is an internal performance optimization for Spark-compatible hex expressions.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.08434% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.67%. Comparing base (1734d4b) to head (4e9f34a).

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/math/hex.rs	71.08%	12 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23688      +/-   ##
==========================================
- Coverage   80.67%   80.67%   -0.01%     
==========================================
  Files        1087     1087              
  Lines      367541   367560      +19     
  Branches   367541   367560      +19     
==========================================
- Hits       296529   296524       -5     
- Misses      53355    53367      +12     
- Partials    17657    17669      +12     

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