fix: honor the volatility of aggregate, window and higher-order functions in Expr::is_volatile

[anishmehta24]

Which issue does this PR close?

• Closes Expr::is_volatile ignores the volatility of aggregate, window and higher-order functions #25504.

Rationale for this change

Expr::is_volatile_node only matched Expr::ScalarFunction, so Expr::is_volatile never saw the volatility that an AggregateUDF, WindowUDF or HigherOrderUDF declares in its signature. Every optimizer rule built on is_volatile (common subexpression elimination, filter pushdown, TopK-through-join, the simplifier, is_repeatable in optimizer::utils) therefore treated a volatile user-defined function of those kinds as deterministic and could deduplicate, move or double-evaluate it. The physical HigherOrderFunctionExpr::is_volatile_node already checks its signature; the logical Expr was the outlier.

What changes are included in this PR?

Expr::is_volatile_node now reads the signature volatility of Expr::ScalarFunction, Expr::AggregateFunction, Expr::WindowFunction (both WindowFunctionDefinition variants, so an aggregate used as a window function keeps its volatility) and Expr::HigherOrderFunction. Every other node is still non-volatile. Expr::is_volatile is unchanged and picks the new cases up through exists.

What is the testing strategy for this PR?

New unit test expr::test::test_is_volatile_aggregate_window_and_higher_order_funcs in datafusion/expr/src/expr.rs: for Immutable, Stable and Volatile signatures it builds an aggregate, a window aggregate, a window UDF and a higher-order function call and checks is_volatile_node / is_volatile, plus the nested lit + agg case. The aggregate/window/HOF assertions fail on main. All built-in functions in these categories are Immutable, so the existing sqllogictests are unaffected.

Are there any user-facing changes?

Optimizer rules now leave volatile user-defined aggregate, window and higher-order functions alone, as they already did for volatile scalar functions. No API change.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.17699% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.38%. Comparing base (710c748) to head (c225aae).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/expr/src/expr.rs	60.17%	45 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25513      +/-   ##
==========================================
- Coverage   82.38%   82.38%   -0.01%     
==========================================
  Files        1138     1138              
  Lines      434313   434421     +108     
  Branches   434313   434421     +108     
==========================================
+ Hits       357805   357879      +74     
- Misses      54876    54908      +32     
- Partials    21632    21634       +2     

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