Merge sorted produces wrong result for lexical categorical

[coastalwhite]

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• 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

s = pl.Series('a', ["c1", "c0"], pl.Categorical(ordering = "lexical"))

df = pl.DataFrame([s.sort()])

merge_sorted = df.lazy().merge_sorted(df.lazy(), "a").collect().get_column("a")
append_sorted = s.append(s).sort()

assert_series_equal(merge_sorted, append_sorted)

Log output

Traceback (most recent call last):
  File "/home/johndoe/Projects/polars/bug.py", line 14, in <module>
    test_merge_sorted_self_parametric(pl.Series('a', ["c1", "c0"], pl.Categorical(ordering = "lexical")))
  File "/home/johndoe/Projects/polars/bug.py", line 12, in test_merge_sorted_self_parametric
    assert_series_equal(merge_sorted, append_sorted)
  File "/home/johndoe/Projects/polars/py-polars/polars/_utils/deprecation.py", line 92, in wrapper
    return function(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johndoe/Projects/polars/py-polars/polars/testing/asserts/series.py", line 117, in assert_series_equal
    _assert_series_values_equal(
  File "/home/johndoe/Projects/polars/py-polars/polars/testing/asserts/series.py", line 197, in _assert_series_values_equal
    raise_assertion_error(
  File "/home/johndoe/Projects/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]:  ['c0', 'c1', 'c0', 'c1']
[right]: ['c0', 'c0', 'c1', 'c1']

Issue description

Expected behavior

Installed versions

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