GH-46683: Add utf8_zfill kernel for sign-aware zero padding

[iabhi4]

What does this PR do?

Adds a new compute kernel utf8_zfill to Arrow Compute module. This kernel zero-pads UTF-8 strings while preserving leading signs.

Why is it needed?

Python's str.zfill() behavior is useful and expected in many data cleaning scenarios. Arrow lacked a direct equivalent.

Implementation details

• Adds Utf8ZFillTransform
• Registers the function in the compute kernel registry
• Adds support to Python bindings and tests
• Includes sign-aware padding logic

Are these changes tested?

• C++ unit tests: TestStringKernels, Utf8ZFill
• Python tests: test_utf8_zfill()
  Ran both tests

• GitHub Issue: [Python] Missing utf8_zfill in pyarrow.compute to support str.zfill behavior #46683

[iabhi4]

@pitrou Just wanted to give you a heads-up that the implementation and tests are ready for review.
Please let me know if I’ve missed anything or if there’s a changelog entry or any documentation I should update as part of this PR.

[AlenkaF]

Thanks for the PR @iabhi4 !

The CI failures look connected. For example:

/arrow/cpp/src/arrow/compute/kernels/scalar_string_internal.h:179:  Check failed: _s.ok() Operation failed: registry->AddFunction(std::move(func))
Bad status: Invalid: In function 'utf8_zfill': description line length exceeds 78 characters

We do not require changelog entry per PR - it is done automatically at the release. As for the documentation, on the Python side, the User Guide is very minimal at this point and so the work would be out of scope of this PR (will keep in mind to maybe add a new issue under #46601).

That being said, I would love if we would have a short example here: https://arrow.apache.org/docs/dev/python/generated/pyarrow.compute.PadOptions.html, even if only for the new utf8_zfill kernel 😊

[iabhi4]

Thanks for the review @AlenkaF, took care of the CI errors and added an Example in the PadOptions docs as suggested

[AlenkaF]

@github-actions crossbow submit preview-docs

[github-actions]

Revision: 7d33c20

Submitted crossbow builds: ursacomputing/crossbow @ actions-08f3ce64cb

Task	Status
preview-docs

[AlenkaF]

Thanks for the updates — the PR looks good!
I just have one minor comment/suggestion, which is optional.

cc @rok / @pitrou mind having a look at the C++ part?

[AlenkaF]

I think a suggestion of mine might have been lost during the review stage. What I meant to propose was setting padding="0" as the default, while still allowing users to override it with any other character. That would align better with the function's name (zero fill) and also provide the desired behaviour without requiring users to explicitly specify the padding each time.