perf: Replace SipHash with foldhash in BoundedWindowAggExec

[neilconway]

Which issue does this PR close?

• Closes: Optimize BoundedWindowAggExec to use foldhash instead of SipHash #23983

Rationale for this change

PartitionWindowAggStates and PartitionBatches were using IndexMap's default SipHash hasher. Switching to foldhash yields a small but measurable performance improvement.

Benchmark below. Note that the improvement is modest in part because linear mode is very inefficient when there are many partitions (#23982); I will send a PR for that separately.

Benchmarks:

• linear, 10,000 partitions: 216.7 ms -> 204.1 ms (~6%)
• linear, 100 partitions: 45.0 ms -> 45.4 ms (within drift)
• sorted, 10,000 partitions: 35.0 ms -> 34.8 ms (control)

What changes are included in this PR?

• Add new benchmark
• Switch from using SipHash to foldhash

Are these changes tested?

Yes, covered by existing tests.

Are there any user-facing changes?

No functional change at all.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.75%. Comparing base (2f25454) to head (ef78b05).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23984      +/-   ##
==========================================
- Coverage   80.75%   80.75%   -0.01%     
==========================================
  Files        1096     1096              
  Lines      373588   373588              
  Branches   373588   373588              
==========================================
- Hits       301687   301685       -2     
+ Misses      53898    53897       -1     
- Partials    18003    18006       +3     

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[neilconway]

@viirya Let me know if you have a chance to take a look :)

[viirya]

LGTM. Swapping the two IndexMap aliases (PartitionWindowAggStates, PartitionBatches) to foldhash RandomState is purely a hasher change — IndexMap preserves insertion order and partition-key equality is decided by PartitionKey's Eq, neither of which the hasher affects, so results are unchanged. Same idea as #23956.

One thing I checked since it's the easy way to break a hasher swap: the two construction sites in bounded_window_agg_exec.rs are correctly updated from IndexMap::new() (only valid for the default hasher) to IndexMap::default(), which works for any Default hasher and builds the same empty map. Good.

Benchmark is honest about the modest win and the linear-mode caveat (#23982). LGTM.

[neilconway]

@viirya Thanks for the review!