perf: skip re-slicing window partition batches with nothing to prune

[neilconway]

Which issue does this PR close?

• Part of BoundedWindowAggExec in Linear mode is slow for many-partitions #23982

Rationale for this change

After each emission, BoundedWindowAggStream prunes the rows of each partition's buffered batch that no longer contribute to any window frame. The prune loop re-sliced every live partition's RecordBatch even when there was nothing to prune, allocating fresh array metadata for an identical batch and then dropping the replaced one.

Profiling the many-partitions benchmark showed prune_state at ~39% of the sparse case, where nearly all live partitions are quiet and almost every slice is a no-op, and ~21% of the dense case, where most slices prune real rows and only the partitions missed by a batch hit the no-op path.

One simple improvement is to leave a partition's buffered batch untouched when the prune count is zero. This avoids the array metadata allocation churn mentioned above.

Benchmarks:

- linear, range, single, 100 part, dense:    44.1 ms  -> 43.8 ms (~noise)
- linear, range, single, 10000 part, dense:  171.1 ms -> 164.8 ms (-3.6%)
- linear, range, single, 32768 part, sparse: 209.4 ms -> 166.1 ms (-20.6%)
- linear, rows,  single, 10000 part, dense:  141.5 ms -> 136.4 ms (-3.7%)
- linear, range, multi,  10000 part, dense:  268.0 ms -> 261.0 ms (-2.6%)
- sorted, range, single, 10000 part:         34.0 ms  -> 34.3 ms (+0.9%)

What changes are included in this PR?

See above.

Are these changes tested?

Covered by existing tests.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (b902256) to head (0ebd494).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24047      +/-   ##
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- Coverage   80.86%   80.85%   -0.01%     
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  Files        1101     1101              
  Lines      375446   375449       +3     
  Branches   375446   375449       +3     
==========================================
- Hits       303598   303582      -16     
- Misses      53758    53774      +16     
- Partials    18090    18093       +3     

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