[BUG] Polars GPU engine returns incorrect nesting for deeply nested list

[Matt711]

Describe the bug
When constructing a nested list column using the GPU engine (via pl.GPUEngine), the result has one less level of nesting compared to the CPU engine. The GPU output is missing a layer of list nesting.

Steps/Code to reproduce bug

import polars as pl

print(
    pl.LazyFrame().select(a=[[[1]]])
    .collect(engine=pl.GPUEngine(raise_on_fail=True))
    .to_dict(as_series=False)
)

print(
    pl.LazyFrame().select(a=[[[1]]])
    .collect()
    .to_dict(as_series=False)
)

CPU
{'a': [[[[1]]]]}

GPU
{'a': [[[1]]]}

Expected behavior
Match polars