Regression: list.sum() inside WhenThen now returns a list

[cmdlineluser]

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": [[1, 2]]})

df.with_columns(sum = pl.when(True).then(pl.col("a").list.sum()))
# shape: (1, 2)
# ┌───────────┬───────────┐
# │ a         ┆ sum       │
# │ ---       ┆ ---       │
# │ list[i64] ┆ list[i64] │
# ╞═══════════╪═══════════╡
# │ [1, 2]    ┆ [3]       │
# └───────────┴───────────┘

Log output

No response

Issue description

Taken from https://stackoverflow.com/questions/78525956/odd-result-from-list-sum-possible-issue

list.sum() inside a WhenThen expression now returns a list.

Expected behavior

Return a number.

Installed versions

--------Version info---------
Polars:               0.20.29
Index type:           UInt32
Platform:             macOS-13.6.1-arm64-arm-64bit
Python:               3.12.2 (main, Feb  6 2024, 20:19:44) [Clang 15.0.0 (clang-1500.1.0.2.5)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.1
pyarrow:              15.0.2
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[orlp]

Can't reproduce on 0.20.23, but can on 0.20.28, a relatively recent regression.