refactor: Do not emit any warnings in tests and treat warnings in tests as errors

[Moritz Potthoff (MoritzPotthoffQC)]

Motivation

Currently, we emit warnings from package code in new polars versions. I think we should not (e.g., as consumers might treat them as errors)

Changes

• Treat warnings as errors, with one exception
• Fix code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a0158ce) to head (74bd653).

Additional details and impacted files

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##              main       #50   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines         1140      1140           
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  Hits          1140      1140           

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[Copilot]

Pull request overview

This PR tightens the project’s warning discipline by treating warnings as errors during pytest runs, and adjusts Polars expression usage to avoid warnings from newer Polars versions.

Changes:

• Configure pytest to treat warnings as errors, with an explicit exception for a known Polars deprecation warning.
• Update a Polars list/array explode usage to pass empty_as_null=True to avoid warning-prone behavior changes.
• Materialize itertools.zip_longest(...) into lists for pytest parametrization stability (avoids iterator-related warnings/collection quirks).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/test_fraction_same.py	Wraps zip_longest argvalues in list(...) for deterministic pytest parametrization without iterator-related warnings.
pyproject.toml	Enables warnings-as-errors for pytest and adds an explicit ignore for a specific Polars deprecation warning.
diffly/comparison.py	Adjusts Expr.explode(...) calls to use empty_as_null=True to align with newer Polars behavior and reduce warnings.

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[Oliver Borchert (borchero)]

Thanks!