[FEA] Support polars.Expr.str.to_decimal in cudf-polars

[brandon-b-miller]

Is your feature request related to a problem? Please describe.
Today on branch-25.08, we lack support for polars.Expr.str. to_decimal. Running the example from the docs yields:

import polars as pl
engine = pl.GPUEngine(raise_on_fail=True)

df = pl.DataFrame(
    {
        "numbers": [
            "40.12",
            "3420.13",
            "120134.19",
            "3212.98",
            "12.90",
            "143.09",
            "143.9",
        ]
    }
).lazy()
df = df.with_columns(numbers_decimal=pl.col("numbers").str.to_decimal())
res = df.collect(engine=engine)
print(res)

 NotImplementedError: dtype=Decimal(precision=None, scale=None)

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. This is blocked on decimal dtypes.

Describe alternatives you've considered
N/A

Additional context
#16480