Lazy select(len()) fails after unique() on first column

[guyrosin]

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

lf = pl.LazyFrame({"a":[1,2,3], "b": [4,5,6]})

lf.unique("a").select(pl.len()).collect()  # <-- fails

lf.unique("b").select(pl.len()).collect()  # <-- succeeds
lf.unique().select(pl.len()).collect()  # <-- succeeds

Log output

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/guy/code/playground/.venv/lib/python3.13/site-packages/polars/lazyframe/frame.py", line 2066, in collect
    return wrap_df(ldf.collect(callback))
                   ~~~~~~~~~~~^^^^^^^^^^
polars.exceptions.ColumnNotFoundError: "a" not found

Issue description

This seems like a regression in polars 1.22.0

Expected behavior

The code should output the frame's length

shape: (1, 1)
┌─────┐
│ len │
│ --- │
│ u32 │
╞═════╡
│ 3   │
└─────┘

Installed versions

--------Version info---------
Polars:              1.22.0
Index type:          UInt32
Platform:            macOS-15.3-arm64-arm-64bit-Mach-O
Python:              3.13.0 (main, Oct  7 2024, 23:47:22) [Clang 18.1.8 ]
LTS CPU:             False

----Optional dependencies----
Azure CLI            <not installed>
adbc_driver_manager  <not installed>
altair               5.5.0
azure.identity       <not installed>
boto3                1.35.93
cloudpickle          3.1.1
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.12.1
fsspec               2025.2.0
gevent               <not installed>
google.auth          2.38.0
great_tables         <not installed>
matplotlib           3.10.0
numpy                2.1.3
openpyxl             <not installed>
pandas               2.2.3
pyarrow              19.0.0
pydantic             2.10.6
pyiceberg            <not installed>
sqlalchemy           2.0.38
torch                2.6.0
xlsx2csv             0.8.4
xlsxwriter           3.2.2

[nameexhaustion]

• Regression from perf: Ensure count query select minimal columns #20923

[ritchie46]

Fix incoming.