refactor: clean up aggregation planning

[2010YOUY01]

Which issue does this PR close?

• Closes Cleanup aggregation planning #24318

Rationale for this change

See issue

What changes are included in this PR?

What is the testing strategy for this PR?

Existing tests.

Are there any user-facing changes?

No.

[2010YOUY01]

This is an existing assumption, and I feel it's slightly confusing.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.80%. Comparing base (4048898) to head (d717c45).

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/aggregates/mod.rs	56.52%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25104      +/-   ##
==========================================
- Coverage   81.80%   81.80%   -0.01%     
==========================================
  Files        1130     1130              
  Lines      417754   417701      -53     
  Branches   417754   417701      -53     
==========================================
- Hits       341754   341698      -56     
+ Misses      55875    55874       -1     
- Partials    20125    20129       +4     

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