tests: strengthen pytest pair-parametrize tests and signature validation

[Code0x58]

Motivation

• Improve coverage and correctness of pair parametrization tests for the alternative/reference utilities by verifying signature preservation and parameter values.
• Make tests more explicit about which implementations are selected for default/double-reference modes.

Description

• Added inspect import and helper _parametrize_values to extract pytest.mark.parametrize values from a decorated test function.
• Renamed and replaced the previous pair-parametrize test with test_pytest_parametrize_pairs_signature_and_parameters which asserts that pytest_parametrize_pairs preserves the original test signature and configures reference/implementation parameter values as expected.
• Updated test reference and implementation callables to have explicit typed signatures (x: int, y: int = 0) -> int and added a pair_test callable to be decorated by pytest_parametrize_pairs for assertion checks.
• Minor docstring cleanup in test_f to reflect default pair parametrization behavior.

Testing

• Ran the test module with pytest -q test_pytest_util.py and all tests in that file passed.
• The new assertions validate function signatures, the contents of parametrized reference and implementation values, and basic execution of the decorated pair test.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8bc0620) to head (d06279a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #6   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          189       189           
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  Hits           189       189           

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