GH-38868: [Python] Add dlpack producer to FixedShapeTensorArray/Scalar

[AntoinePrv]

Rationale for this change

Have the same coverage as to_numpy.

What changes are included in this PR?

• FixedShapeTensorArray.__dlpack__ explicitly calling to_tensor
• FixedShapeTensorArray.__dlpack_device__ (defaulted)
• FixedShapeTensorScalar.__dlpack__ explicitly calling to_tensor
• FixedShapeTensorScalar.__dlpack_device__ calling C++ device extraction on the underlying array.

If GH-51122 get merged first, I'll add the consuming side here, otherwise if this one gets merged first, I'll add it there.

Are these changes tested?

Yes a few since this does not introduce new logic.

Are there any user-facing changes?

Additions only.

• GitHub Issue: [C++][Python] DLPack on FixedShapeTensorArray/FixedShapeTensorScalar #38868

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[Copilot]

🟡 Changes recommended

It introduces a backward-incompatible change to the public Array.__dlpack__ signature and contains a newly added incorrect docstring for FixedShapeTensorScalar.__dlpack__.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR extends PyArrow’s DLPack producer support to FixedShapeTensorArray and FixedShapeTensorScalar, with accompanying tests to match the existing to_numpy-level coverage, and refactors NumPy version gating in DLPack tests.

Changes:

• Add a reusable requires_numpy_version() marker and apply it to version-dependent DLPack tests.
• Implement __dlpack__ for FixedShapeTensorArray and both __dlpack__ / __dlpack_device__ for FixedShapeTensorScalar.
• Add new DLPack roundtrip tests for permuted fixed-shape tensor arrays and scalars.

File summaries

File	Description
python/pyarrow/tests/test_dlpack.py	Adds NumPy-version gating helper and new tests for fixed-shape tensor array/scalar DLPack export behavior.
python/pyarrow/scalar.pxi	Adds DLPack producer/device hooks for FixedShapeTensorScalar.
python/pyarrow/array.pxi	Adds DLPack producer for FixedShapeTensorArray and changes Array.__dlpack__ signature.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🔵 Needs a closer look

FixedShapeTensorScalar.__dlpack_device__ can dereference null storage for invalid scalars, risking a crash instead of raising a Python exception.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

python/pyarrow/scalar.pxi:1616

• __dlpack_device__ dereferences ext.value / storage.value without checking scalar validity. For a null (invalid) FixedShapeTensorScalar this can be a null pointer and crash instead of raising a Python exception. Please guard on self.is_valid before using the storage scalar.

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

[AntoinePrv]

@AlenkaF @rok @pitrou this one is ready too