fix: Crash/incorrect group_by/n_unique on categoricals created by (q)cut

[nameexhaustion]

Fixes #15981.

We recently added the categories upfront to make them deterministic, but we forgot to ensure that the fast unique setting was set properly:

>>> pl.Series([1]).cut([1, 2, 3])
shape: (1,)
Series: '' [cat]
[
	"(-inf, 1]"
]
>>> pl.Series([1]).cut([1, 2, 3]).n_unique()
4

As a note, it looks like CategoricalChunkedBuilder automatically sets fast unique to true by default:

polars/crates/polars-core/src/chunked_array/logical/categorical/builder.rs

Lines 195 to 206 in d5cf038

	pub fn finish(self) -> CategoricalChunked {
	// SAFETY: keys and values are in bounds
	unsafe {
	CategoricalChunked::from_keys_and_values(
	&self.name,
	&self.cat_builder.into(),
	&self.categories.into(),
	self.ordering,
	)
	}
	.with_fast_unique(true)
	}