perf: null-free fast path for COUNT(DISTINCT) primitive accumulator

[viirya]

Which issue does this PR close?

• Closes COUNT(DISTINCT <primitive>) misses a null-free fast path in its update loop #23955.

Rationale for this change

PrimitiveDistinctCountAccumulator::update_batch does a per-element validity check on every row even when the array has no nulls. Its own merge_batch already inserts wholesale from the values buffer, so the same fast path applies to update_batch. Inspired by the null-free fast paths added to percentile_cont/median in #23954.

What changes are included in this PR?

• When null_count() == 0, insert directly from the values buffer (self.values.extend(arr.values().iter().copied())) instead of the per-element Option check.
• Unit test asserting the fast and general paths agree (a dense array and a null-containing array with the same non-null values yield the same distinct count).

Benchmarks

count_distinct benchmark, null-free 8192-row batches:

count_distinct i64 80% distinct   -67.0%  (54.9 -> 18.1 us)
count_distinct i64 99% distinct   -66.0%  (54.9 -> 18.7 us)
count_distinct u32 80% distinct   -66.9%  (53.9 -> 17.8 us)
count_distinct u32 99% distinct   -65.3%  (53.3 -> 18.5 us)
count_distinct i32 80% distinct   -66.9%  (54.1 -> 17.9 us)
count_distinct i32 99% distinct   -66.2%  (53.2 -> 18.0 us)

The bitmap-backed cases (u8/i8/u16/i16) and the grouped-accumulator cases don't use this path and are unchanged, as expected.

Are these changes tested?

Yes — new unit test; existing count-distinct tests pass.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.69%. Comparing base (b0b9dae) to head (034c216).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23956      +/-   ##
==========================================
- Coverage   80.69%   80.69%   -0.01%     
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  Files        1095     1095              
  Lines      372529   372556      +27     
  Branches   372529   372556      +27     
==========================================
+ Hits       300626   300643      +17     
- Misses      53942    53946       +4     
- Partials    17961    17967       +6     

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[neilconway]

lgtm! Nice perf win for such a narrowly scoped change.

[viirya]

Thanks @neilconway for review