Support negative scale decimals in decimal-to-integer conversions

[shifluxxc]

Which issue does this PR close?

• part of Allow log/pow on negative scale decimals #19250

Rationale for this change

Decimal values with negative scale represent multiplication by a power of ten, but the decimal-to-integer conversion helpers previously rejected such values. This caused valid negative-scale decimals to fail during evaluation.

What changes are included in this PR?

Update decimal32_to_i32, decimal64_to_i64, and decimal128_to_i128 to correctly handle negative scales by multiplying

Are these changes tested?

Yes. Existing unit tests were updated and new cases were added to validate correct behavio

[shifluxxc]

@alamb can you review the changes please .

[alamb]

Thank you for this PR @shifluxxc

I think the code looks good but I found the current test coverage in the .slt confusing

I think we need tests that try different types of decimals (including with negative scale), perhaps something like this (also adding Decimal32(9,-2), etc)? Though I don't fully comprehend the issue yet

# Test converting decimals to integers

statement ok
create table decimals as
select
  arrow_cast(column1, 'Decimal32(9,2)') as dec32
from values
    ('123.45'),
    ('-678.90'),
    ('0.00');

query R
select log(dec32, 10) from decimals;
----
0.478127782968
NaN
0

[alamb]

these tests pass on main as well (so I don't think they cover the intended usecase)