PanicException in Polars 1.18 on mode() with string-dtype

[thomas-privat]

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

s = polars.Series("a", ["x", "y"])
s.mode()

Log output

thread '<unnamed>' panicked at crates\polars-core\src\series\mod.rs:533:18:
invalid from_physical(String) for Binary
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "D:\****\.venv\Lib\site-packages\polars\series\utils.py", line 106, in wrapper
    return s.to_frame().select_seq(f(*args, **kwargs)).to_series()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\*****\.venv\Lib\site-packages\polars\dataframe\frame.py", line 9347, in select_seq
    return self.lazy().select_seq(*exprs, **named_exprs).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\****.venv\Lib\site-packages\polars\lazyframe\frame.py", line 2043, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: invalid from_physical(String) for Binary

Issue description

no exeption is produced, with numeric data e.g. polars.Series("a", [1, 2]), nor with polars version 1.17

Expected behavior

no panic exception

Installed versions

--------Version info---------
Polars:              1.18.0
Index type:          UInt32
Platform:            Windows-2019Server-10.0.17763-SP0
Python:              3.12.6 (tags/v3.12.6:a4a2d2b, Sep  6 2024, 20:11:23) [MSC v.1940 64 bit (AMD64)]
LTS CPU:             False

----Optional dependencies----
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           3.9.2
nest_asyncio         <not installed>
numpy                2.1.3
openpyxl             <not installed>
pandas               2.2.3
pyarrow              18.1.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0

[cmdlineluser]

This is fixed on main.

• fix: Fix mode panicking for String dtype #20458