ENH: fix HalfCauchy scipy mapping and add dedicated tests (towards #22)

[kunal14901]

Reference Issues/PRs

See #22

What does this implement/fix? Explain your changes.

This PR fixes scipy parameter mapping for HalfCauchy and adds dedicated regression tests.

Changes made:

• Updated HalfCauchy._get_scipy_param to pass beta as scipy keyword argument scale.

  • before: return [beta], {}
  • after: return [], {"scale": beta}

• Added skpro/distributions/tests/test_halfcauchy.py with focused tests for:

  • scalar parity against scipy.stats.halfcauchy for pdf, cdf, and ppf
  • support behavior for negative inputs (x < 0)
  • broadcasting and shape/index/columns behavior with DataFrame inputs

Does your contribution introduce a new dependency? If yes, which one?

No, this PR does not introduce any new dependency.

What should a reviewer concentrate their feedback on?

• Correctness of scipy adapter mapping in _get_scipy_param
• Correctness/style of the new HalfCauchy tests
• Any preferred convention for distribution-specific test modules

Did you add any tests for the change?

Yes.

Added:

• skpro/distributions/tests/test_halfcauchy.py

Validation:

• targeted test file passed
• test_all_distrs.py filtered to HalfCauchy passed (103 selected tests)

Any other comments?

Submitted as part of my ESoC 2026 application contribution towards #22.
Happy to iterate quickly on feedback.

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've earned
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG].

For new estimators

• Not applicable (this PR fixes adapter mapping and adds tests for an existing distribution)
• Not applicable
• Not applicable

[Copilot]

Pull request overview

This PR fixes the SciPy adapter parameter mapping for HalfCauchy so that beta is passed as SciPy’s scale kwarg (instead of being interpreted as loc via a positional arg), and adds focused regression tests to prevent reintroducing the issue.

Changes:

• Corrected HalfCauchy._get_scipy_param to return {"scale": beta} as keyword arguments.
• Added test_halfcauchy.py to verify scalar parity with scipy.stats.halfcauchy (pdf, cdf, ppf), support behavior for x < 0, and DataFrame broadcasting/shape preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
skpro/distributions/halfcauchy.py	Fixes the SciPy parameter mapping so beta is passed as scale.
skpro/distributions/tests/test_halfcauchy.py	Adds regression tests covering scalar parity, support for negative inputs, and DataFrame broadcasting/shape behavior.

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[kunal14901]

Hi @fkiraly and @felipeangelimvieira , thanks for reviewing.
This PR is a focused bug fix towards #22:

fixes HalfCauchy scipy parameter mapping (scale=beta)
adds dedicated regression tests for scipy parity and broadcast/shape behavior
All targeted tests pass locally, and I am happy to make any changes quickly