GH-49826: [Python] Return NotImplemented from Scalar/Array arithmetic dunders for unsupported types

[alex-anast]

Rationale for this change

In pyarrow 24.0.0, Scalar and Array gained arithmetic dunder methods (#32007) that unconditionally dispatch to pyarrow.compute.call_function. When the other operand is an unrecognized type, _pack_compute_args raises TypeError instead of returning NotImplemented. This prevents Python from falling back to the other operand's reflected methods (__radd__, __rsub__, etc.), breaking downstream libraries that relied on this protocol.

What changes are included in this PR?

• Adds a _compute_binary_op helper in scalar.pxi that wraps call_function in a try/except TypeError and returns NotImplemented on failure. This follows the same pattern already used by Scalar.__eq__ and Array.__eq__.
• Updates all binary arithmetic and bitwise dunders on both Scalar (scalar.pxi) and Array (array.pxi) to use this helper.
• Adds parametrized tests in test_scalars.py and test_array.py verifying that reflected operators on custom types are correctly invoked.

Are these changes tested?

Yes. New parametrized tests (test_dunders_return_notimplemented_for_unknown_types) cover all 10 binary operators for both Scalar and Array. The bug was also manually reproduced against pyarrow 24.0.0 to confirm the tests exercise the right code path.

Are there any user-facing changes?

Yes. Scalar and Array arithmetic dunders now return NotImplemented instead of raising TypeError when the other operand is not a recognized Arrow/NumPy type. This restores the pre-24.0.0 behavior where Python would fall back to the other operand's reflected method.

• GitHub Issue: [Python] Scalar arithmetic dunders raise TypeError instead of returning NotImplemented #49826
  Closes [Python] Scalar arithmetic dunders raise TypeError instead of returning NotImplemented #49826

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.