Use pytest-beartype-tests plugin

[adamtheturtle]

This PR adds the pytest-beartype-tests dev dependency and removes redundant pytest_collection_modifyitems / @beartype wiring from conftest where it duplicated the plugin.

The plugin registers via pytest11 and applies @beartype to collected test functions, matching the previous local hook behavior (see the upstream README).

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Note

Low Risk
Low risk: changes are limited to test tooling/dependencies and removal of a redundant pytest collection hook, with no production code impact.

Overview
Test-time runtime type checking is now provided via the pytest-beartype-tests plugin instead of a local pytest_collection_modifyitems hook in tests/conftest.py.

The PR adds pytest-beartype-tests to dev dependencies and uv.lock, removes the now-unused beartype wiring from conftest.py, and adjusts config/metadata (including dropping pytest_collection_modifyitems from vulture ignores and adding an empty [dependency-groups] section).

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