Combination of when-then statement with over and mapping_strategy="join" runs forever

[ElisabethBrockhausQC]

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

df = pl.DataFrame(
    {"a": [True, True], "b": ["foo", "bar"], "c": [1, 2]},
    schema={"a": pl.Boolean, "b": pl.String, "c": pl.UInt16},
)

df.with_columns(
    pl.when(pl.col("a"))
    .then(pl.col("b").over("c", mapping_strategy="join"))
    .otherwise(pl.col("b"))
    .alias("d")
)

Log output

Issue description

The given code example runs forever.

Expected behavior

We would expect some kind of exception indicating that the schema of the expressions in .then() and in .otherwise() are not compatible.

Installed versions

--------Version info---------
Polars:              1.16.0
Index type:          UInt32
Platform:            macOS-15.2-arm64-arm-64bit
Python:              3.12.7 | packaged by conda-forge | (main, Oct  4 2024, 15:57:01) [Clang 17.0.6 ]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               5.5.0
boto3                1.35.73
cloudpickle          3.1.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.12.0
fsspec               2024.10.0
gevent               <not installed>
google.auth          2.36.0
great_tables         <not installed>
matplotlib           3.9.3
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             3.1.5
pandas               2.2.3
pyarrow              18.1.0
pydantic             2.10.3
pyiceberg            <not installed>
sqlalchemy           2.0.35
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0