concat_list creates a nested list

[tim-x-y-z]

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

>>> df=pl.DataFrame({"a": [[1,2,3], [3,4,5]], "b": [0.5, 0.1], "group":[1,2]})
>>> df.explode("a").with_columns(new_col=pl.col("a") * pl.col("b")).groupby("group").agg(pl.concat_list("a"), pl.first("b"), pl.concat_list("new_col"))
shape: (2, 4)
┌───────┬─────────────────┬─────┬───────────────────────┐
│ group ┆ a               ┆ b   ┆ new_col               │
│ ---   ┆ ---             ┆ --- ┆ ---                   │
│ i64   ┆ list[list[i64]] ┆ f64 ┆ list[list[f64]]       │
╞═══════╪═════════════════╪═════╪═══════════════════════╡
│ 1     ┆ [[1], [2], [3]] ┆ 0.5 ┆ [[0.5], [1.0], [1.5]] │
│ 2     ┆ [[3], [4], [5]] ┆ 0.1 ┆ [[0.3], [0.4], [0.5]] │
└───────┴─────────────────┴─────┴───────────────────────┘

Issue description

In some ways, related to #9948, as my goal is to multiply a list by another column value. This stack overflow questions did the same: https://stackoverflow.com/questions/74372173/python-polars-how-to-multiply-each-element-in-a-list-with-a-value-in-a-differen
However, the api seems to have cahnge since then as list is not callable anymore, being now a namespace only seemingly.
I assumed the replacement was concat_list which doesn't work as expected. I may be wrong that this is not what concat_list is for, but from the example here it seemed it is.

Expected behavior

Just returns a list of the object that it is concat. ie in this case, just return list[f64]

Installed versions

>>> pl.show_versions()
--------Version info---------
Polars:              0.18.7
Index type:          UInt32
Platform:            macOS-13.4.1-x86_64-i386-64bit
Python:              3.11.4 (main, Jun 20 2023, 16:59:59) [Clang 14.0.3 (clang-1403.0.22.14.1)]

----Optional dependencies----
adbc_driver_sqlite:  <not installed>
connectorx:          0.3.2a7
deltalake:           <not installed>
fsspec:              <not installed>
matplotlib:          3.7.1
numpy:               1.25.1
pandas:              2.0.3
pyarrow:             12.0.1
pydantic:            1.10.11
sqlalchemy:          1.4.49
xlsx2csv:            <not installed>
xlsxwriter:          <not installed>```

</details>

[stinodego]

We changed the behavior of agg some time ago.

You can do:

import polars as pl

df = pl.DataFrame({"a": [[1, 2, 3], [3, 4, 5]], "b": [0.5, 0.1], "group": [1, 2]})

result = (
    df.explode("a")
    .with_columns(new_col=pl.col("a") * pl.col("b"))
    .groupby("group")
    .agg("new_col")
)

No need to manually convert to a list.