refactor: extract aggregate physical planning

[1cbyc]

Which issue does this PR close?

• Part of Move large LogicalPlan arms out of DefaultPhysicalPlanner::map_logical_node_to_physical #26300.

Rationale for this change

DefaultPhysicalPlanner::map_logical_node_to_physical contains the complete aggregate planning implementation inline. Moving this arm behind a focused method makes the top-level logical-plan dispatcher easier to navigate and reduces conflict pressure around planner changes.

What changes are included in this PR?

This PR extracts only the Aggregate arm into a private plan_aggregate method. Schema validation, grouping expression lowering, async expression planning, repartition selection, and partial/final aggregate construction are unchanged.

What is the testing strategy for this PR?

• cargo test -p datafusion --lib physical_planner::tests
• cargo clippy -p datafusion --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check

The existing physical planner tests cover aggregate schema checks, grouping sets, repartitioning, aliases, and scalar subqueries; no new behavior is introduced.

Are there any user-facing changes?

No. This is an internal refactor with no public API or behavior change.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.29630% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.87%. Comparing base (5009f42) to head (23a4cd8).

Files with missing lines	Patch %	Lines
datafusion/core/src/physical_planner.rs	96.29%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26403   +/-   ##
=======================================
  Coverage   82.87%   82.87%           
=======================================
  Files        1153     1153           
  Lines      457101   457124   +23     
  Branches   457101   457124   +23     
=======================================
+ Hits       378818   378841   +23     
+ Misses      55268    55267    -1     
- Partials    23015    23016    +1     

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[2010YOUY01]

Thank you!

I've let AI checked this logic has been moved without change