Replace duplicated scalar/array broadcast match in math_funcs with a shared helper #6982

What is the problem the feature request solves?

The same match that turns a (ColumnarValue, ColumnarValue) pair into two equal-length ArrayRefs is pasted into several math functions in native/spark-expr/src/math_funcs/:

• checked_arithmetic.rs checked_arithmetic_internal (L348-L359), plus a similar block at L186-L195 in the same file
• div.rs spark_decimal_div_internal (L87-L102)
• modulo_expr.rs checked_float_modulo (L91-L102)
• wide_decimal_binary_expr.rs WideDecimalBinaryExpr::evaluate (L196-L212)

The copies are identical apart from variable names, so there is no drift today, but they all need to change together if scalar broadcasting behaviour changes.

Describe the potential solution

Replace the copies with ColumnarValue::values_to_arrays(&[left, right]) from DataFusion, or, if the scalar-scalar length-1 behaviour needs to stay explicit, a single broadcast_pair(l, r) -> Result<(ArrayRef, ArrayRef)> helper in math_funcs, and call it from each site.

Additional context

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