Refactor Alternatives pytest parameter selection into testable helpers

[Code0x58]

Motivation

• Make the logic that selects implementation callables for pytest parametrization deterministic and directly unit-testable instead of embedded inline in decorators.
• Allow explicit verification of behavior across boolean options (only_default, double_reference) and keep pytest-friendly callable IDs.

Description

• Extracted the implementation-selection logic from Alternatives.pytest_parametrize(...) into a new private method Alternatives._select_parametrize_implementations(...) and updated the decorator to use it.
• Extracted the pair-selection logic from Alternatives.pytest_parametrize_pairs(...) into a new private method Alternatives._select_parametrize_pairs(...) and updated the decorator to use it.
• Preserved existing semantics for only_default, double_reference, exclusion of the reference implementation in pair mode when appropriate, and use of underlying callables for readable pytest IDs.
• Added focused tests in test_pytest_util.py that assert the exact callable lists returned by the new helpers across all boolean combinations.

Testing

• Ran uv run --dev pytest and all tests passed: 33 passed.
• Ran uv run --dev ruff format --check . and formatting checks passed.
• Attempted uv run --dev pyrefly check . but pyrefly is not available in this environment so it was not run successfully.
• Ran mypy . which reported pre-existing type-check issues in alternative.py unrelated to these changes.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@6594c5d). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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