avg over a decimal with a negative scale panics with "attempt to divide by zero" #24898

Describe the bug

DecimalAverager::try_new (datafusion/functions-aggregate-common/src/utils.rs) computes 10^sum_scale and 10^target_scale with pow_wrapping(scale as u32). For a negative scale the exponent wraps to a huge value and the wrapped power is 0. DecimalAverager::avg then evaluates target_mul.div_wrapping(sum_mul) with sum_mul == 0 and panics.

All four decimal widths are affected (Decimal32, Decimal64, Decimal128, Decimal256); the plain aggregate, the grouped aggregate and avg(DISTINCT ...) all go through the same helper.

To Reproduce

SELECT avg(x) FROM (VALUES (arrow_cast(1, 'Decimal128(10, -2)'))) t(x);

thread 'main' panicked at library/core/src/num/mod.rs:475:5:
attempt to divide by zero

(arrow_array::arithmetic::div_wrapping called from DecimalAverager::avg, datafusion/functions-aggregate-common/src/utils.rs.)

Expected behavior

avg of 100 and 200 at scale -2 (i.e. 10 000 and 20 000) returns 15000.00 as Decimal128(14, 2), the type Avg::return_type already declares for that input.

Additional context

The two factors are only ever used as the ratio 10^(target_scale - sum_scale), which is a plain non-negative exponent whenever the target scale is not smaller than the input scale (already enforced).

Found while running a corpus of extreme-value literals against a debug build of datafusion-cli.