fix(python): Address incorrect selector & col expansion

[alexander-beedie]

Closes #19740.

Should resolve as an intersection, not a union.

Example

import polars.selectors as cs
import polars as pl

df = pl.DataFrame({
    "a": [1],
    "b": [2]
})

Before

df.select(cs.by_name("a") & pl.col("b"))
# shape: (1, 2)
# ┌─────┬─────┐
# │ a   ┆ b   │
# │ --- ┆ --- │
# │ i64 ┆ i64 │
# ╞═════╪═════╡
# │ 1   ┆ 2   │
# └─────┴─────┘

After

df.select(cs.by_name("a") & pl.col("b"))
# shape: (0, 0)
# ┌┐
# ╞╡
# └┘