DataFrame.pivot returns incorrect results when (single) index is struct

[MarcoGorelli]

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

In [1]: df = pl.DataFrame({'a': [1, 2], 'b': [{'a': 1}, {'a': 1}], 'c': ['x', 'y']})

In [2]: df
Out[2]:
shape: (2, 3)
┌─────┬───────────┬─────┐
│ a   ┆ b         ┆ c   │
│ --- ┆ ---       ┆ --- │
│ i64 ┆ struct[1] ┆ str │
╞═════╪═══════════╪═════╡
│ 1   ┆ {1}       ┆ x   │
│ 2   ┆ {1}       ┆ y   │
└─────┴───────────┴─────┘

In [3]: df.pivot(index='b', values='a', columns='c')
Out[3]:
shape: (2, 3)
┌───────────┬──────┬──────┐
│ b         ┆ x    ┆ y    │
│ ---       ┆ ---  ┆ ---  │
│ struct[1] ┆ i64  ┆ i64  │
╞═══════════╪══════╪══════╡
│ {null}    ┆ 1    ┆ null │
│ {null}    ┆ null ┆ 2    │
└───────────┴──────┴──────┘

Log output

No response

Issue description

here's the cause (and solution!) #14048 (comment)

Expected behavior

I think it should be

shape: (1, 3)
┌───────────┬──────┬──────┐
│ b         ┆ x    ┆ y    │
│ ---       ┆ ---  ┆ ---  │
│ struct[1] ┆ i64  ┆ i64  │
╞═══════════╪══════╪══════╡
│ {1}       ┆ 1    ┆ 2    │
└───────────┴──────┴──────┘

Installed versions

--------Version info---------
Polars:               0.20.5
Index type:           UInt32
Platform:             Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:               3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fsspec:               2023.12.2
gevent:               <not installed>
hvplot:               0.9.1
matplotlib:           <not installed>
numpy:                1.26.3
openpyxl:             <not installed>
pandas:               1.3.4
pyarrow:              15.0.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           2.0.25
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[ritchie46]

Is this still the case?

[MarcoGorelli]

yes - got a few things on my plate this week but will try to get to it