Panic when doing an invalid melt

[kszlim]

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

pl.LazyFrame({"a": ["x","y"], "b": [["test", "test2"], ["test3", "test4"]]}).melt().collect()

Log output

PanicException: called `Result::unwrap()` on an `Err` value: InvalidOperation(ErrString("cannot cast List type (inner: 'String', to: 'String')"))

Issue description

Shouldn't panic, but perhaps should throw SchemaException?

Expected behavior

Shouldn't panic, but perhaps should throw SchemaException?

Installed versions

--------Version info---------
Polars:               1.0.0-beta.1
Index type:           UInt32
Platform:             Linux-5.10.218-186.862.x86_64-x86_64-with-glibc2.26
Python:               3.11.7 (main, Dec  5 2023, 22:00:36) [GCC 7.3.1 20180712 (Red Hat 7.3.1-17)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2024.5.0
gevent:               <not installed>
great_tables:         0.7.0
hvplot:               <not installed>
matplotlib:           3.8.4
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.2
pyarrow:              16.1.0
pydantic:             2.7.2
pyiceberg:            <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>```

</details>

[ravikanthreddy89]

@kszlim output on my machine (Mac M2, Python 3.11.7, Polars 0.20.31)

[kszlim]

You're on an older version, also it's not clear if that type unification makes sense.