assert_series_equal with categorical_as_str does not work on nested Categoricals

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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
from polars.testing import assert_series_equal

# Global
with pl.StringCache():
    s1 = pl.Series(["c0"], dtype=pl.Categorical)
    s2 = pl.Series(["c1"], dtype=pl.Categorical)

    s_global = pl.DataFrame([s1, s2]).to_struct("col0")

# Local
s1 = pl.Series(["c0"], dtype=pl.Categorical)
s2 = pl.Series(["c1"], dtype=pl.Categorical)
s_local = pl.DataFrame([s1, s2]).to_struct("col0")

assert_series_equal(s_global, s_local, categorical_as_str=True)

Log output

Traceback (most recent call last):
  File "/home/stijn/code/polars/py-polars/repro.py", line 16, in <module>
    assert_series_equal(s_global, s_local, categorical_as_str=True)
  File "/home/stijn/code/polars/py-polars/polars/testing/asserts/series.py", line 105, in assert_series_equal
    _assert_series_values_equal(
  File "/home/stijn/code/polars/py-polars/polars/testing/asserts/series.py", line 174, in _assert_series_values_equal
    raise_assertion_error(
  File "/home/stijn/code/polars/py-polars/polars/testing/asserts/utils.py", line 12, in raise_assertion_error
    raise AssertionError(msg) from cause
AssertionError: Series are different (exact value mismatch)
[left]:  [{'column_0': 'c0', 'column_1': 'c1'}]
[right]: [{'column_0': 'c0', 'column_1': 'c1'}]

Issue description

With categorical_as_str=True, categoricals should be treated as their string representation. This is currently not the case for nested types.

The offending code is here:

polars/py-polars/polars/testing/asserts/series.py

Lines 127 to 132 in 9bfa30c

	# Handle categoricals
	if categorical_as_str:
	if left.dtype == Categorical:
	left = left.cast(String)
	if right.dtype == Categorical:
	right = right.cast(String)

Clearly this will not work for nested types.

Expected behavior

Assertion should pass

Installed versions

main