perf: Use agg DistinctHandling in join optimization

[neilconway]

Which issue does this PR close?

• N/A; see perf: Convert inner joins to semi joins when equivalent #22652 and feat: DISTINCT handling for Aggregate UDFs #25288 for context

Rationale for this change

If we can prove that parts of a query are insensitive to duplicates, the optimizer apply various simplifications, like replacing inner joins with semi-joins and removing unused outer-join inputs.

The join analysis was previously conservative and assumed that all aggregate expressions are duplicate sensitive. Since #25288 added a framework for classifying how an aggregate treats duplicate values, we can now apply that framework to optimize joins more effectively.

What changes are included in this PR?

• Extend EliminateJoin to recognize Aggregate plan nodes whose aggregate expressions all declare DistinctHandling::Insensitive
• Guard duplicate-insensitivity propagation against volatile expressions and subqueries, to avoid changing query results
• Refactor code to share the existing volatility/subquery check with UnionsToFilter.
• Add tests

What is the testing strategy for this PR?

Existing tests pass; new tests added.

Are there any user-facing changes?

Some query plans might change (usually for the better).

[neilconway]

FYI @mkleen @adriangb @jayzhan211

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.10619% with 81 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.92%. Comparing base (b0b5471) to head (ab9e061).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/optimizer/src/eliminate_join.rs	73.24%	14 Missing and 66 partials ⚠️
datafusion/optimizer/src/utils.rs	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25385      +/-   ##
==========================================
- Coverage   81.93%   81.92%   -0.01%     
==========================================
  Files        1136     1136              
  Lines      428779   429449     +670     
  Branches   428779   429449     +670     
==========================================
+ Hits       351319   351836     +517     
- Misses      56446    56503      +57     
- Partials    21014    21110      +96     

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