Panic when combining .list.explode() with .over(mapping_strategy="join")

[atombear]

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

on polars 1.29.0

df = pl.DataFrame({"x": [[1.0], [2.0]]}).with_row_index("idx")
e = pl.col("x").list.explode().over("idx", mapping_strategy="join")
df.with_columns(e).equals(df)  # True
df.select(pl.col("x").list.diff()) # no problem
df.select(e.list.diff())  # panic

Log output

group_by keys are sorted; running sorted key fast path

thread '<unnamed>' panicked at crates/polars-plan/src/dsl/function_expr/list.rs:94:55:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
---------------------------------------------------------------------------
PanicException                            Traceback (most recent call last)
Cell In[2], line 5
      3 df.with_columns(e).equals(df)  # True
      4 df.select(pl.col("x").list.diff()) # no problem
----> 5 df.select(e.list.diff())

File ~/miniforge3/envs/impressionism-env/lib/python3.10/site-packages/polars/dataframe/frame.py:9657, in DataFrame.select(self, *exprs, **named_exprs)
   9557 def select(
   9558     self, *exprs: IntoExpr | Iterable[IntoExpr], **named_exprs: IntoExpr
   9559 ) -> DataFrame:
   9560     """
   9561     Select columns from this DataFrame.
   9562 
   (...)
   9655     └──────────────┘
   9656     """
-> 9657     return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)

File ~/miniforge3/envs/impressionism-env/lib/python3.10/site-packages/polars/_utils/deprecation.py:93, in deprecate_streaming_parameter.<locals>.decorate.<locals>.wrapper(*args, **kwargs)
     89         kwargs["engine"] = "in-memory"
     91     del kwargs["streaming"]
---> 93 return function(*args, **kwargs)

File ~/miniforge3/envs/impressionism-env/lib/python3.10/site-packages/polars/lazyframe/frame.py:2224, in LazyFrame.collect(self, type_coercion, _type_check, predicate_pushdown, projection_pushdown, simplify_expression, slice_pushdown, comm_subplan_elim, comm_subexpr_elim, cluster_with_columns, collapse_joins, no_optimization, engine, background, _check_order, _eager, **_kwargs)
   2222 # Only for testing purposes
   2223 callback = _kwargs.get("post_opt_callback", callback)
-> 2224 return wrap_df(ldf.collect(engine, callback))

PanicException: called `Option::unwrap()` on a `None` value

Issue description

this worked in 1.20

Expected behavior

pl.col("x") and pl.col("x").list.explode().over("idx", mapping_strategy="join") should be equivalent in this case.

Installed versions

--------Version info---------
Polars:              1.29.0
Index type:          UInt32
Platform:            macOS-15.4.1-x86_64-i386-64bit
Python:              3.10.5 | packaged by conda-forge | (main, Jun 14 2022, 07:03:09) [Clang 13.0.1 ]
LTS CPU:             False

i see this on linux as well

[coastalwhite]

Doing some initial investigation, this seems to be a regression from the recent fixes to .explode() + .over(). It is a mismatch between the type check result from .over and what is actually done in the engines. I need to have it a bit more clear what the signature of .over should be in this case.