Disallow setting `default` for files/paths

For example, this ain't gonna work:

def predict(self, file: Path = Input(default=Path("image.png")):

The thing consuming the schema (e.g. Replicate) can't get that default. We should just not allow this until we've figured out a better solution here.

As an aside: just setting the default to string is allowed but shouldn't be. Maybe we should validate default values to check they're actually the correct type.

Reported by @yael-vinker (thank you!)