Columns converted to `str` if there is one `Category` column and `coerce = True`

Describe the bug A clear and concise description of what the bug is.

[x] I have checked that this issue has not already been reported.
[x] I have confirmed this bug exists on the latest version of pandera.
[ ] (optional) I have confirmed this bug exists on the main branch of pandera.

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Code Sample, a copy-pastable example

import datetime
from pandera.polars import Field # type: ignore
from pandera.polars import DataFrameModel # type: ignore
from pandera.engines.polars_engine import Category
from typing import Optional

import polars as pl

class MyModel(DataFrameModel):
    a: datetime.datetime = Field(description="some description")
    b: Category = Field(description="some description", dtype_kwargs={"categories": ["a", "b", "c"]})

df = pl.DataFrame({"a": [datetime.datetime.now(), datetime.datetime.now()], "b": ["a", "b"]})
schema = MyModel.to_schema()
schema.strict = True
schema.coerce = True # -> this is what causes the conversion of `a` to string
print(schema.validate(df)) # BOOM!

Exception: pandera.errors.SchemaError: expected column 'a' to have type Datetime(time_unit='us', time_zone=None), got String

When checking the df in debug mode, we see that it's after the coerce of column b that column a becomes converted to str.

Expected behavior

The dataframe should've been validated and no types changed in this case. Column b should be coerced to pl.Category.

Desktop (please complete the following information):

OS: macOS 14.6.1 Python 3.12.4 polars-lts-cpu 1.6.0 pandera 0.20.3

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Additional context

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unionai-oss / pandera