chore: adjust size accounting for min_max

[comphead]

Which issue does this PR close?

• Part of feat: analyze memory usage for size functions #23393 .

  Rationale for this change

  SlidingMinAccumulator::size and SlidingMaxAccumulator::size only reported
  the stack size of their ScalarValue field plus its heap payload, ignoring the
  memory held by the underlying MovingMin / MovingMax sliding-window buffers.
  For windowed MIN/MAX over string or list data, the two per-element stacks
  can hold megabytes of ScalarValue payload that the memory pool was never
  told about, understating accumulator memory usage.

  What changes are included in this PR?

  • Add a private heap_size(elem_heap) method to MovingMin<T> and MovingMax<T>
    that reports the two stack buffers' capacity in bytes plus each stored
    element's heap payload.
  • Factor the shared implementation into a moving_stacks_heap_size free helper
    so the two types stay in sync.
  • Include the buffer bytes in SlidingMinAccumulator::size and
    SlidingMaxAccumulator::size via the new method.

  Are these changes tested?

  Yes. Two new unit tests in datafusion/functions-aggregate/src/min_max.rs:

  • moving_min_max_heap_size_i32 — fixed-width T, verifies buffer-only
    accounting with and without pushed elements.
  • moving_min_max_heap_size_counts_elems — T = String, verifies each of the
    two slots in a (T, T) pair contributes independently to the heap payload
    (mirroring the two independent Clones made by push).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.75510% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (8393fd3) to head (1afc8ef).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions-aggregate/src/min_max.rs	87.75%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23899   +/-   ##
=======================================
  Coverage   80.65%   80.65%           
=======================================
  Files        1092     1092           
  Lines      371106   371188   +82     
  Branches   371106   371188   +82     
=======================================
+ Hits       299323   299396   +73     
+ Misses      53937    53933    -4     
- Partials    17846    17859   +13     

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[comphead]

Thanks @Jefffrey for the review!