`.str.split` coerces `dtype` to `object`

**Describe the issue**: The `.str.split` method coerces `dtype` from `string` to `object`. This behavior is also inconsistent with `pandas` (if that matters). **Minimal complete verifiable example**: Code ```python import dask.dataframe as dd import pandas as pd data = {"c": ["a,b,c", "d,e,f", "g,h,i"]} # `pandas` df = pd.DataFrame(data, dtype="string[pyarrow]") print(df.dtypes) print(df["c"].str.split(",", n=1, expand=True).dtypes) # `dask` ddf = dd.from_pandas(df) print(ddf.dtypes) print(ddf["c"].str.split(",", n=1, expand=True).dtypes) ``` Output ``` c string[pyarrow] dtype: object 0 string[pyarrow] 1 string[pyarrow] dtype: object c string[pyarrow] dtype: object 0 object 1 object dtype: object ``` **Environment**: - Dask version: 2025.3.0 - Python version: 3.10.16 - Operating System: Ubuntu 24.04.2 LTS - Install method (conda, pip, source): pip