Projection pushdown with hive partitions may not be respected

[nameexhaustion]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl
from pathlib import Path


root = Path(".env/data2")

dfs = [
    pl.DataFrame({"x": 1}),
    pl.DataFrame({"x": 2}),
]

paths = [
    root / "a=1/b=1/data.bin",
    root / "a=2/b=2/data.bin",
]

[
    [paths[i].parent.mkdir(exist_ok=True, parents=True), dfs[i].write_parquet(paths[i])]
    for i in range(len(dfs))
]

lf = pl.scan_parquet(root / "**/*.bin")
lf = lf.select("x", "a")

print(lf.collect())

Observe the select was not respected:

shape: (2, 3)
┌─────┬─────┬─────┐
│ x   ┆ a   ┆ b   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 1   ┆ 1   ┆ 1   │
│ 2   ┆ 2   ┆ 2   │
└─────┴─────┴─────┘

Log output

No response

Issue description

#15573 removes the projection node after projection pushdown into the parquet reader, which reveals that the parquet reader was not applying the projection properly on the hive partition columns.

Expected behavior

shape: (2, 2)
┌─────┬─────┐
│ x   ┆ a   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 1   │
│ 2   ┆ 2   │
└─────┴─────┘

Installed versions

main @ 8a6bf4b

[deanm0000]

duplicate of this I put some notes in there that might be helpful.

[nameexhaustion]

duplicate of this I put some notes in there that might be helpful.

Thanks for the triage!