test: end-to-end reproducer for aggregating batches with stricter nested nullability than the declared schema

[alamb]

Which issue does this PR close?

• Part of GroupedHashAggregateStream::emit throws ArrowError: column types must match schema types #24069 (reproducer only; the tests fail on main by design, so this does not close the issue).

Rationale for this change

get a reproducer so we can tell when the error happened

What changes are included in this PR?

An end-to-end reproducer as a basis for working out a root-cause fix.

Are these changes tested?

The tests are the change. They currently fail (and one panics) on main, demonstrating the bug.

cargo test --test core_integration -- nested_nullability

And it fails like this

thread 'sql::aggregates::nested_nullability::array_agg_struct_from_stricter_batches' (14311250) panicked at /Users/andrewlamb/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/arrow-array-59.2.0/src/array/list_array.rs:272:54:
called `Result::unwrap()` on an `Err` value: InvalidArgumentError("ListArray expected data type Struct(\"colA\": Boolean) got Struct(\"colA\": non-null Boolean) for \"item\"")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: Context("Join Error", External(JoinError::Panic(Id(65), "called `Result::unwrap()` on an `Err` value: InvalidArgumentError(\"ListArray expected data type Struct(\\\"colA\\\": Boolean) got Struct(\\\"colA\\\": non-null Boolean) for \\\"item\\\"\")", ...)))

---- sql::aggregates::nested_nullability::array_agg_struct_from_stricter_batches_with_spilling stdout ----

I also ran the same test on the 54.1.0 branch:

andrewlamb@Andrews-MacBook-Pro-3:/tmp/54.1.0$ git cherry-pick 6a5d04fead7fb6ecf8c53ad3a725c176794d3aef
Auto-merging datafusion/core/tests/sql/aggregates/mod.rs
[detached HEAD 782a17c405] Add end-to-end reproducer for aggregating batches stricter than the declared schema
 Date: Mon Aug 10 19:14:37 2026 -0400
 2 files changed, 180 insertions(+)
 create mode 100644 datafusion/core/tests/sql/aggregates/nested_nullability.rs
andrewlamb@Andrews-MacBook-Pro-3:/tmp/54.1.0$ cargo test --test core_integration -- nested_nullability

And it fails the same way

...
thread 'sql::aggregates::nested_nullability::array_agg_struct_from_stricter_batches' (14314739) panicked at /Users/andrewlamb/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/arrow-array-58.3.0/src/array/list_array.rs:272:54:
called `Result::unwrap()` on an `Err` value: InvalidArgumentError("ListArray expected data type Struct(\"colA\": Boolean) got Struct(\"colA\": non-null Boolean) for \"item\"")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: Context("Join Error", External(JoinError::Panic(Id(65), "called `Result::unwrap()` on an `Err` value: InvalidArgumentError(\"ListArray expected data type Struct(\\\"colA\\\": Boolean) got Struct(\\\"colA\\\": non-null Boolean) for \\\"item\\\"\")", ...)))
...

Are there any user-facing changes?

No.