Python(feat): pytest parameterize support

[alexluck-sift]

Summary

• Add nested-parametrize step trees to the pytest plugin: parametrized tests cluster under one shared parent (e.g. test_foo), and stacked @pytest.mark.parametrize decorators nest outer-to-inner.
• Stash each item's parametrize path in pytest_collection_modifyitems and sort items by it so siblings share a single open parent step; a new autouse _parametrize_parents fixture diffs the path against the open stack and opens/closes parents lazily.
• Drain any open parametrize parents on module_substep teardown and pytest_sessionfinish so the report's step assertions hold.
• Tests: 5 pytester-driven cases (single parametrize, stacked decorators, fixture params, module boundary isolation, full parent chain) using a FakeReportContext that records the resulting step tree.

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-570/

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