LazyFrame has incorrect schema following aggregation of an exploded list column expression

[nsfinkelstein]

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

import polars as pl

df = pl.LazyFrame({ 'a': [0, 0, 1, 2], 'b': [['a'], ['b'], ['c'], ['d', 'e']]})
df1 = df.group_by('a').agg(pl.col('b').explode())
df2 = df1.collect()

print(df1.collect_schema())
print(df2.collect_schema())
assert df1.collect_schema() == df2.collect_schema(), 'collecting should not change schema'

Log output

Schema([('a', Int64), ('b', String)])
Schema([('a', Int64), ('b', List(String))])
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
Cell In[1], line 9
      7 print(df1.collect_schema())
      8 print(df2.collect_schema())
----> 9 assert df1.collect_schema() == df2.collect_schema(), 'collecting should not change schema'

AssertionError: collecting should not change schema

Issue description

The LazyFrame seems to lose track of the correct schema when aggregating an exploded list expression. This is resolved upon calling collect, however it can lead to some difficult bugs if there are lazy operations following the aggregation.

Expected behavior

Calling collect should not change the schema, which in this case would lead to no assertion error. In particular the LazyFrame schema should know that c has a List type.

Installed versions

--------Version info---------
Polars:              1.12.0
Index type:          UInt32
Platform:            Linux-5.10.219-208.866.amzn2.x86_64-x86_64-with-glibc2.35
Python:              3.11.9 (main, Sep 23 2024, 17:32:51) [GCC 11.4.0]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          3.1.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.9.0
gevent               <not installed>
great_tables         0.13.0
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             <not installed>
pandas               2.2.3
pyarrow              17.0.0
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           2.0.35
torch                2.5.0+cu124
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[ritchie46]

Can you take a look at this one @nameexhaustion?