fix: avoid divide-by-zero in avg for negative-scale decimals

[breken-ai]

Which issue does this PR close?

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

Rationale for this change

avg over a decimal column with a negative scale panics with attempt to divide by zero instead of returning a result. For example, averaging 10000 and 20000 stored as Decimal128(10, -2) should return 15000.00 as Decimal128(14, 2) (the type Avg::return_type already declares), but the query task panics. The plain, grouped and DISTINCT aggregates all go through the same helper.

What changes are included in this PR?

DecimalAverager::try_new previously computed 10^sum_scale and 10^target_scale separately with pow_wrapping(scale as u32). A negative scale becomes a huge u32 exponent, the wrapped power is 0, and avg then divides target_mul by that zero sum_mul.

The two factors were only ever used as the ratio 10^(target_scale - sum_scale), so the averager now stores that single multiplier:

• the scale difference is computed in i16, so it is exact for any pair of i8 scales;
• a negative difference (target scale smaller than the input scale) returns the existing Arithmetic Overflow in AvgAccumulator error, as the old target_mul >= sum_mul check did;
• the power uses pow_checked, so an unrepresentable multiplier also returns that error instead of wrapping.

avg multiplies the sum by the stored multiplier; the precision validation is unchanged.

What is the testing strategy for this PR?

Added avg_decimal_negative_scale to datafusion/sqllogictest/test_files/aggregate.slt, covering the plain aggregate (value and Decimal128(14, 2) result type), a grouped aggregate and avg(DISTINCT ...) over Decimal128(10, -2) values.

• On main (1d9be2e10) with only the new test: cargo test --profile ci -p datafusion-sqllogictest --test sqllogictests -- aggregate.slt fails at the new query with task 11 panicked with message "attempt to divide by zero".
• With the fix, the same command passes, and the full cargo test --profile ci -p datafusion-sqllogictest --test sqllogictests run passes (524/524 files).
• cargo test --profile ci -p datafusion-functions-aggregate-common -p datafusion-functions-aggregate passes, cargo clippy -p datafusion-functions-aggregate-common -p datafusion-functions-aggregate --all-targets --all-features -- -D warnings is clean, and cargo fmt --all -- --check passes.

Are there any user-facing changes?

Yes: avg over negative-scale decimals returns the average instead of panicking. No public API changes (the changed fields are private).

AI disclosure: this fix was found, written and tested by the breken-ai agent (Breken). Happy to answer questions about the scale arithmetic during review.

[AdamGS]

doesn't the ok() here loses some error information?

[breken-ai]

Yes, it did. pow_checked returns an ArrowError that says what overflowed, and .ok() discarded it. Fixed in ac0e791: try_new now keeps the error.

let scale_mul = ten.pow_checked(scale_diff as u32).map_err(|e| {
    exec_datafusion_err!(
        "Arithmetic Overflow in AvgAccumulator: cannot rescale from scale {sum_scale} to {target_scale}: {e}"
    )
})?;

The message still starts with Arithmetic Overflow in AvgAccumulator. No .slt or Rust test matches on the old text. The from_usize(10) case (can't happen for i32/i64/i128/i256) is reported as an internal error again, as it was before this PR.

I also added unit tests in functions-aggregate-common/src/utils.rs:

• decimal_averager_negative_scale: Decimal128(10, -2) → (14, 2) rescales a sum of 300 at scale -2 by 10^4 and averages to 1500000.
• decimal_averager_unrepresentable_multiplier_keeps_cause: try_new(-128, 38, 127) needs 10^255, which doesn't fit in an i128. The error now reads ... cannot rescale from scale -128 to 127: Arithmetic overflow: Overflow happened on: 10 ^ 255. On the previous head (60067ef) the same test fails with just Execution error: Arithmetic Overflow in AvgAccumulator.

cargo test -p datafusion-functions-aggregate-common: 62 passed. cargo clippy -p datafusion-functions-aggregate-common --all-targets --all-features -- -D warnings and cargo fmt --all -- --check are clean, and the branch still merges cleanly into main.