fix: respect volatility for all function expressions

[1fanwang]

Which issue does this PR close?

Closes #25504.

Rationale for this change

Repeated calls to a volatile higher-order UDF can reuse the first call's result. Comparing two calls can also return true without invoking the function. Both queries now execute each call.

What changes are included in this PR?

The volatility check reads the declared signature for aggregate, window and higher-order expressions, as it already does for scalar expressions. This includes aggregates used as window functions.

What is the testing strategy for this PR?

The regression registers a counter UDF and runs both queries through the SQL engine. A separate check covers all function kinds, each volatility level, and aliased expressions.

Testing Done

Query	Before	After
Two counter calls in a projection	(0, 0), one invocation	(0, 1), two invocations
Equality between two counter calls	true, no invocations	false, two invocations

I used Rust 1.98.1 on macOS arm64 with the installed macOS 15.4 SDK. From the PR checkout, these commands run the same regression source against the baseline and the fix:

export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX15.4.sdk
export CARGO_BUILD_JOBS=2

git worktree add -b 1fannnw/repro-udf-volatility \
  ../datafusion-before-25504 42f3888ca2fc82da8548dc4f13533af7af9b5a9f
git diff 42f3888ca2fc82da8548dc4f13533af7af9b5a9f HEAD \
  -- datafusion/core/tests/user_defined |
  git -C ../datafusion-before-25504 apply -

cargo test --locked --profile ci \
  --manifest-path ../datafusion-before-25504/Cargo.toml \
  --target-dir target/volatility-before \
  -p datafusion --test user_defined_integration volatility -- --nocapture

cargo test --locked --profile ci \
  --target-dir target/volatility-after \
  -p datafusion --test user_defined_integration volatility -- --nocapture

Raw logs

Before the fix, the projection returned:

+-------+--------+
| first | second |
+-------+--------+
| 0     | 0      |
+-------+--------+
calls=1

The equality comparison returned:

+-------+
| equal |
+-------+
| true  |
+-------+
calls=0

After the fix, the projection returned:

+-------+--------+
| first | second |
+-------+--------+
| 0     | 1      |
+-------+--------+
calls=2

The equality comparison returned:

+-------+
| equal |
+-------+
| false |
+-------+
calls=2

• Local code review completed

Are there any user-facing changes?

Queries affected by the incorrect volatility check return the results of separate function calls. There is no public API change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.38%. Comparing base (42f3888) to head (78cbf21).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #25511    +/-   ##
========================================
  Coverage   82.37%   82.38%            
========================================
  Files        1138     1138            
  Lines      434081   434322   +241     
  Branches   434081   434322   +241     
========================================
+ Hits       357588   357814   +226     
- Misses      54864    54875    +11     
- Partials    21629    21633     +4     

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[neilconway]

There's also #25513 for the same bug