GH-50312: [Python] Fix UUID extension type round-trip to pandas returning bytes

[parker-cassar]

Rationale for this change

Converting a Table with an arrow.uuid extension column to pandas currently produces a column of bytes instead of uuid.UUID objects. This happens because UuidType does not implement to_pandas_dtype(), so Table.to_pandas() falls back to the storage type (fixed_size_binary(16)) and produces bytes. The bug occurs even without a Parquet roundtrip.

Note: the original issue suggested this might be specific to Python 3.14 but I tested on Python versions 3.10 - 3.14 and still had the issue since UuidType has never implemented to_pandas_dtype().

What changes are included in this PR?

Added UuidType.to_pandas_dtype(): returns a dtype wrapper implementing __from_arrow__, which delegates to to_pylist() since UuidScalar.as_py() already produces uuid.UUID objects.

Are these changes tested?

Yes. Added test_uuid_roundtrip which covers pandas DataFrame with a UUID column -> pyarrow Table -> Parquet on disk -> pyarrow Table -> pandas DataFrame. The final conversion is what this PR fixes.

Are there any user-facing changes?

Yes. Table.to_pandas() now returns uuid.UUID for arrow.uuid columns instead of bytes.

• GitHub Issue: [Python][Parquet] FIXED_LEN_BYTE_ARRAY fails to cast to UUID on Python 3.14 / Nightly builds #50312

[github-actions]

⚠️ GitHub issue #50312 has been automatically assigned in GitHub to PR creator.

[GiTaDi-CrEaTe]

Hey @parker-cassar, thanks so much for jumping on this so quickly. Just out of curiosity—since this delegates to to_pylist(), do you think this pattern might end up being useful for other Arrow extension types down the way???