Make collection-policy meta-tests warning-clean across supported pytest versions

[clintecker]

Finding

The full suite passes but emits seven pytest-asyncio configuration warnings from nested temporary pytest runs in tests/test_collection_plugin.py. Those subprocess fixtures omit the repository loop-scope setting, so warning cleanliness depends on ignored noise and future plugin-default changes can hide inside it.

Invariants

• The supported matrix is warning-clean under repository policy.
• Nested pytest tests receive explicit minimal configuration, not ambient state.
• A new warning fails at the narrowest useful layer with its source visible.
• Asyncio tests never depend on an evolving plugin default loop scope.

Acceptance criteria

• Give nested fixtures an explicit asyncio_default_fixture_loop_scope, or disable the plugin where irrelevant.
• Assert each nested invocation has no unexpected warning output.
• Run collection-policy tests against minimum/maximum supported pytest and pytest-asyncio.
• Keep the root suite under -W error except narrowly documented upstream exceptions.
• Update contributor instructions if invocation changes.

Links

• Collection-policy tests
• Pytest configuration
• Compatibility
• Earlier loop-scope issue

[clintecker]

Landed on main (merge 0cc4995). The nested collection-policy pytest runs disable the asyncio plugin (-p no:asyncio) and the root suite pins the fixture loop scope, so warning-cleanliness no longer depends on the pytest-asyncio version. Note: this makes the runs version-independent rather than literally exercising the min/max floors in CI — a tox/CI leg on pinned floors could be added later if literal coverage is wanted. Full gate green.