GH-51046: [Python][Interchange] Preserve the categorical ordered flag in from_dataframe

[Kayvan-Zahiri]

Rationale for this change

from_dataframe builds the dictionary array without the is_ordered flag the producer reports, so an ordered categorical column comes back unordered. A pyarrow table with dictionary(int32, string, ordered=True) does not survive its own round trip, and an ordered pandas Categorical loses its ordering on the way in. The pandas consumer passes the flag through (pd.Categorical(values, categories=categories, ordered=categorical["is_ordered"])), and our own producer reports it, so pyarrow is the only side dropping it.

What changes are included in this PR?

categorical_column_to_dictionary passes ordered=categorical["is_ordered"] to DictionaryArray.from_arrays. Indices, dictionary and null handling are untouched.

Are these changes tested?

Yes. test_pyarrow_roundtrip_categorical is now parametrized over ordered True and False, and the ordered cases fail without the change: assert table.equals(result) sees ordered=1 going in and ordered=0 coming out. I also fixed a copy-paste in test_pandas_roundtrip_categorical, which read describe_categorical from the result column twice, so its is_ordered assertion compared the result with itself and could never fail.

Are there any user-facing changes?

An ordered dictionary column stays ordered through from_dataframe. No API change.

I used an AI assistant while finding and writing this. I checked the behavior and the tests myself before opening the PR.

• GitHub Issue: [Python][Interchange] from_dataframe drops the categorical ordered flag #51046

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See also:

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[github-actions]

⚠️ GitHub issue #51046 has been automatically assigned in GitHub to PR creator.