fix: Preserve name when casting to Enum

[mcrumiller]

Resolves #14318.

import polars as pl

dtype1 = pl.Enum(["a", "b"])
dtype2 = pl.Enum(["a", "b", "Null"])

s = pl.Series(name="x", values=["a", "b", None], dtype=dtype1)
print(s.name)

x

[stinodego]

Thanks, just one comment about the test!

[stinodego]

Could you just make a separate test test_cast_enum_name_retention? That should make things a bit cleaner.

[stinodego]

Thanks!