max_horizontal Gets Wrong Value For Decimal Columns With Different Scales

[Purpl3Koala]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

from decimal import Decimal
import polars as pl

df = pl.DataFrame({
    'a': [25, 30, 35],
})

df = df.with_columns(
    pl.lit(str(Decimal('1.000000000001'))).cast(pl.Decimal).alias('b'),
    pl.lit(str(Decimal('3.3333333'))).cast(pl.Decimal).alias('c'),
    pl.lit(str(Decimal('2.2222222'))).cast(pl.Decimal).alias('d')
)

df = df.with_columns(
    pl.max_horizontal('b', 'c').alias('max_horizontal1'),
    pl.max_horizontal('c', 'd').alias('max_horizontal2')
)

print(df)

Log output

No response

Issue description

Creating a column with max_horizontal() across two decimal columns with different scales has a result with the wrong scale.
Max_horizontal between two decimal columns with the same scale has the expected result.

My apologies if I have confused scale vs precision here.

Expected behavior

Max_horizontal() outputs a column with the correct maximum value with the appropriate scale.

Installed versions

--------Version info---------
Polars:               0.20.26
Index type:           UInt32
Platform:             Windows-10-10.0.19044-SP0
Python:               3.11.5 (tags/v3.11.5:cce6ba9, Aug 24 2023, 14:38:34) [MSC v.1936 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  0.11.0
cloudpickle:          3.0.0
connectorx:           0.3.2
deltalake:            0.17.1
fastexcel:            0.10.4
fsspec:               2023.12.2
gevent:               24.2.1
hvplot:               0.9.2
matplotlib:           <not installed>
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.2
pyarrow:              16.0.0
pydantic:             2.7.1
pyiceberg:            0.6.0
pyxlsb:               <not installed>
sqlalchemy:           2.0.29
torch:                <not installed>
xlsx2csv:             0.8.2
xlsxwriter:           3.2.0