perf: preallocate RowsGroupColumn buffers in take_n

[saadtajwar]

Which issue does this PR close?

• Closes Preallocate RowsGroupColumn buffers during take_n rebuilds #23994

Rationale for this change

Currently RowsGroupColumn's take_n method knows the remaining number of rows and bytes after taking the first n group rows, but when invoking empty_rows passes in 0 for the row capacity and data capacity parameters instead of the known remainders, potentially causing unnecessary copying and allocating

What changes are included in this PR?

Using the known remaining number of rows and bytes in the invocation to empty_rows

Are these changes tested?

Yes

Are there any user-facing changes?

No

[saadtajwar]

@kosiew - ready for review!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.89%. Comparing base (882d906) to head (9a26f49).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24070      +/-   ##
==========================================
+ Coverage   80.88%   80.89%   +0.01%     
==========================================
  Files        1102     1102              
  Lines      375813   376184     +371     
  Branches   375813   376184     +371     
==========================================
+ Hits       303971   304318     +347     
- Misses      53741    53755      +14     
- Partials    18101    18111      +10     

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[kosiew]

@saadtajwar
Looks good overall. I left one non-blocking suggestion to make the preallocation behavior more directly exercised by the test.

[kosiew]

The test does a good job of confirming that rebuilding works correctly, but I do not think it distinguishes the new preallocation logic from the previous empty_rows(0, 0) path. The outer-null FixedSizeList has a shorter row encoding, but that row is emitted rather than retained. The retained inner-null and non-null Int32 rows appear to have the same encoded length.

Could we change or add retained rows with different encoded sizes, perhaps using FixedSizeList<Utf8> values with unequal payload lengths? That would exercise the lengths().skip(n).sum() preallocation calculation more directly.

[saadtajwar]

Thanks - pushed a change to exercise this directly! Please let me know your thoughts!