fix: Preserve generic in ChunkedArray.type

[dangotbanned]

Discovered during narwhals-dev/narwhals#2007

I worked around it with this, but seemed simple enough to upstream:

def _type(self: pa.ChunkedArray[pa.Scalar[_DataType_CoT]]) -> _DataType_CoT:
    if TYPE_CHECKING:
        return self[0].type
    return self.type

[dangotbanned]

Found quite a lot of other things in (narwhals-dev/narwhals#2007) that could help improve ChunkedArray support.

I had seen #169, but that narwhals PR revealed many more that could be widened from Array

[dangotbanned]

@zen-xu do you have a preference on resolving these?
https://github.com/search?q=repo%3Anarwhals-dev%2Fnarwhals+path%3A%2F%5Enarwhals%5C%2F_arrow%5C%2F%2F+%22Incomplete%22&type=code&rgh-link-date=2025-02-19T09%3A31%3A12Z

1. I could make a general issue to track all the things we need fixing in (https://github.com/narwhals-dev/narwhals)?
   i. There are some that are related
   ii. but all of these were only documented as of (fix(typing): Resolve all mypy & pyright errors for _arrow narwhals-dev/narwhals#2007)
2. Just jump straight to the PRs?