feat: Raise informative error on Unknown unnest

[ritchie46]

closes #19812

>>> df = pl.DataFrame({"a": [[1, 2, 3]]})
>>> df.select(pl.col("a").list.to_struct().struct.unnest()
---------------------------------------------------------------------------
InvalidOperationError                     Traceback (most recent call last)
Cell In[3], line 2
     1 df = pl.DataFrame({"a": [[1, 2, 3]]})
----> 2 df.select(pl.col("a").list.to_struct().struct.unnest())

File ~/code/polars/py-polars/polars/dataframe/frame.py:9029, in DataFrame.select(self, *exprs, **named_exprs)
  8929 def select(
  8930     self, *exprs: IntoExpr | Iterable[IntoExpr], **named_exprs: IntoExpr
  8931 ) -> DataFrame:
  8932     """
  8933     Select columns from this DataFrame.
  8934 
  (...)
  9027     └──────────────┘
  9028     """
-> 9029     return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)

File ~/code/polars/py-polars/polars/lazyframe/frame.py:2029, in LazyFrame.collect(self, type_coercion, predicate_pushdown, projection_pushdown, simplify_expression, slice_pushdown, comm_subplan_elim, comm_subexpr_elim, cluster_with_columns, collapse_joins, no_optimization, streaming, engine, background, _eager, **_kwargs)
  2027 # Only for testing purposes
  2028 callback = _kwargs.get("post_opt_callback", callback)
-> 2029 return wrap_df(ldf.collect(callback))

InvalidOperationError: expected 'struct' got an unknown data type

This means there was an operation of which the output data type could not be determined statically.
Try setting the output data type for that operation.

Hint: set 'upper_bound' for 'list.to_struct'.

Resolved plan until failure:

   ---> FAILED HERE RESOLVING 'select' <---
DF ["a"]; PROJECT */1 COLUMNS; SELECTION: None

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 22 lines in your changes missing coverage. Please review.

Project coverage is 79.33%. Comparing base (245d549) to head (fa982fe).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-plan/src/plans/conversion/expr_expansion.rs	8.33%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19830      +/-   ##
==========================================
- Coverage   79.33%   79.33%   -0.01%     
==========================================
  Files        1548     1548              
  Lines      214193   214215      +22     
  Branches     2460     2460              
==========================================
  Hits       169937   169937              
- Misses      43698    43720      +22     
  Partials      558      558              

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