Add ColumnarValue::to_array_variant method (follow-up to #22784)

[nathanb9]

Which issue does this PR close?

Follow-up to #22784, addressing a review comment from @alamb suggesting the expand_if_scalar helper would make a good method on ColumnarValue.

Rationale for this change

map.rs had a private expand_if_scalar helper that expands a Scalar to an array while leaving an Array as is, keeping the value wrapped as a ColumnarValue. This is generally useful for functions that thread a ColumnarValue but need to assume a non-scalar input, so it belongs on ColumnarValue rather than in one function file.

What changes are included in this PR?

• Add ColumnarValue::to_array_variant(&self, num_rows), which returns a ColumnarValue guaranteed to be the Array variant. It reuses the existing to_array, so a Scalar is expanded to num_rows and an Array is returned unchanged (no length validation).
• Remove the private expand_if_scalar from map.rs and call the new method instead.

Are these changes tested?

Yes. Added a unit test for to_array_variant covering scalar expansion, array pass-through, and that an existing array is not length validated. Existing map unit tests and sqllogictests continue to pass.

Are there any user-facing changes?

Adds one public method to ColumnarValue. No behavior changes.

[Jefffrey]

personally i'm not sure about introducing this method, as it seems to goes against the purpose of a ColumnarValue in representing scalars and arrays distinctly 🤔

it would be better for the use case to manually unwrap to just an array if it can only operate on arrays instead of doing this conversion