[Python] Fix DeprecationWarnings in PyArrow tests

[AlenkaF]

Describe the enhancement requested

I have stumbled upon this deprecation warning in test_pandas.py:

pyarrow/tests/test_pandas.py::TestConvertMisc::test_category
  /Users/alenkafrim/Repos/arrow/python/pyarrow/tests/test_pandas.py:3078: Pandas4Warning: Constructing a Categorical with a dtype and values containing non-null entries not in that dtype's categories is deprecated and will raise in a future version.
    'cat_strings_with_na': pd.Categorical(v1 * repeats,

pyarrow/tests/test_pandas.py::TestConvertMisc::test_category_implicit_from_pandas
  /Users/alenkafrim/Repos/arrow/python/pyarrow/tests/test_pandas.py:3104: Pandas4Warning: Constructing a Categorical with a dtype and values containing non-null entries not in that dtype's categories is deprecated and will raise in a future version.
    pd.Categorical(['a', 'b', 'c'], categories=['a', 'b']),

pyarrow/tests/test_pandas.py::TestConvertMisc::test_category_implicit_from_pandas
  /Users/alenkafrim/Repos/arrow/python/pyarrow/tests/test_pandas.py:3105: Pandas4Warning: Constructing a Categorical with a dtype and values containing non-null entries not in that dtype's categories is deprecated and will raise in a future version.
    pd.Categorical(['a', 'b', 'c'], categories=['a', 'b'],

There might be more we should address.

Component(s)

Python

[shashbha14]

take

[IbrahimHassan-arch]

hello , This issue piqued my interest, And I would like to work on this issue. Can I get more information and guidance on this topic? I want to get familiar to the project. I am also participation in GSOC 2026 in the PyArrow improvements for astronomy project. Can I get some guidance to be familiar with codebase? @AlenkaF

[GujaLomsadze]

PR 49271 addressing this looks stalled (no activity since March, changes requested by @AlenkaF unaddressed). I'd like to pick it up - taking AlenkaF's suggested approach: making the null explicit (['a','b',None]) instead of relying on the deprecated silent-drop, and dropping the redundant cat_strings_with_na. Will open a PR.