POLARS_NEW_MULTIFILE bug when selecting hive column

[kszlim]

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
import os

df = pl.DataFrame({"id": [1,1,1,2,2,2], "c": [1, 2, 3, 1, 2, 3]})
df.write_parquet("dataset", partition_by="id")

os.environ["POLARS_NEW_MULTIFILE"] = "1"

print(pl.scan_parquet("dataset", hive_partitioning=True).select(pl.col("id").unique()).collect())

Log output

polars.exceptions.ColumnNotFoundError: "id" not found

Issue description

This should correctly return

Expected behavior

┌─────┐
│ id  │
│ --- │
│ i64 │
╞═════╡
│ 1   │
│ 2   │
└─────┘

Installed versions

--------Version info---------
Polars:              1.20.0
Index type:          UInt32
Platform:            macOS-15.1-arm64-arm-64bit
Python:              3.12.7 (main, Oct 16 2024, 07:12:08) [Clang 18.1.8 ]
LTS CPU:             False

----Optional dependencies----
Azure CLI            <not installed>
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                <not installed>
openpyxl             <not installed>
pandas               <not installed>
pyarrow              <not installed>
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>