chore: type `constructor_eager` as `ConstructorEager` instead of `Constructor`

[MarcoGorelli]

In here

https://github.com/narwhals-dev/narwhals/blob/9353ef9f681f032feee777fabbb8db8ae98aa79d/tests/utils.py#L20

we have Constructor

The task is here it to:

• make ConstructorEager, with IntoDataFrame instead of IntoFrame
• in tests that have constructor_eager: Any, use instead constructor_eager: ConstructorEager. You may need to import it, see other tests for examples

[lucianosrp]

I'll take this one if no one is on it 😊