Extract shared list-array construction from the split_* functions in split.rs #6983

What is the problem the feature request solves?

In native/spark-expr/src/string_funcs/split.rs, the four generic split functions end with the same block of code: finish the values builder, pick Utf8/LargeUtf8 from O::IS_LARGE, build the item field, and construct a GenericListArray with the offsets.

• L305-L327 (regex scalar)
• L338-L360 (split_sql_generic_scalar)
• L374-L397 (split_sql_generic_scalar_array)
• L415-L441 (split_sql_generic_array)

They differ only in the per-row call and in how the null buffer is produced (cloned input nulls vs NullBufferBuilder). A fix to the list construction, such as the item field's nullability, has to be made in four places.

Describe the potential solution

Extract fn finish_split_list<O: OffsetSizeTrait>(offsets: Vec<O>, values: GenericStringBuilder<O>, nulls: Option<NullBuffer>) -> ColumnarValue and call it from all four functions. Optionally, also factor the per-row loop into a generic driver so each variant only supplies its null predicate and push call.

Additional context

This is small and a good first issue. Line references are pinned to commit 7d8d9b1.