Panic on DataFrame.pivot when using common aggregate function on string data

[jbarlow83]

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'],
        "b": ['4', '5', '6'],
        "c": ['7', '8', '9'],
    }
)
df.pivot(index='a', columns='b', values='c', aggregate_function='min')

Log output

thread 'polars-24' panicked at crates/polars-core/src/series/iterator.rs:90:9:
assertion `left == right` failed: impl error
  left: 3
 right: 1

PanicException                            Traceback (most recent call last)
/tmp/ipykernel_216518/2344724553.py in ?()
----> 1 df2.pivot(index='a', columns='b', values='c', aggregate_function='max')

~/Documents/MARs/Scripts/.venv/lib/python3.11/site-packages/polars/_utils/deprecation.py in ?(*args, **kwargs)
    223         @wraps(function)
    224         def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
    225             if len(args) > num_allowed_args:
    226                 issue_deprecation_warning(msg, version=version)
--> 227             return function(*args, **kwargs)

~/Documents/MARs/Scripts/.venv/lib/python3.11/site-packages/polars/dataframe/frame.py in ?(self, values, index, columns, aggregate_function, maintain_order, sort_columns, separator)
   7767         else:
   7768             aggregate_expr = aggregate_function._pyexpr
   7769 
   7770         return self._from_pydf(
-> 7771             self._df.pivot_expr(
   7772                 index,
   7773                 columns,
   7774                 values,

PanicException: assertion `left == right` failed: impl error
  left: 3
 right: 1

Issue description

When attempting to pivot on string data, panics occur when an aggregate function is used on string inputs.

Expected behavior

Instead of panicking, raise an exception stating that aggregate functions like max/max are not capable of handling non-numeric data types, or something like that.

Note that somewhere around 0.20.20, aggregate functions worked on strings.

Installed versions

--------Version info---------
Polars:               0.20.31
Index type:           UInt32
Platform:             Linux-6.9.3-76060903-generic-x86_64-with-glibc2.35
Python:               3.11.9 (main, Apr  6 2024, 17:59:24) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           0.3.2
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         1.5.8
numpy:                2.0.0
openpyxl:             3.1.4
pandas:               2.2.2
pyarrow:              16.1.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             0.8.2
xlsxwriter:           3.2.0

[ritchie46]

I cannot reproduce on main?

[MarcoGorelli]

it reproduces on 0.20.31, but is fixed in the pre-release. looks like it's "fixed itself"?

might be worth just adding a little test to make sure it doesn't regress