Make unexpected warnings test failures

[derek73]

Summary

• Adds filterwarnings = ["error"] to the pytest config, so any unexpected warning during the suite is a test failure. Warnings are load-bearing API in 2.0 (the multi-word-entry UserWarning, the facade deprecation warnings), and without this a warning regression is invisible — the eight spurious warnings on restoring a 1.4 Constants pickle shipped all the way to the final PR Post-rc1 API polish: StrEnum Role, Parser conveniences, dead-vocabulary repair #290 review this way, with 2,393 tests green around it.
• Zero fallout: the suite is already warning-disciplined (every legitimately-warning path asserts via pytest.warns or a local simplefilter("error") block), so no ignore entries and no test changes were needed. The list stays a bare "error" — the narrowest possible end state.
• Guard verified non-vacuous both ways: a scratch reproduction of the historical pickle bug now fails the run (with the correct --rootdir; a scratchpad-rooted run silently skips pyproject.toml — noted for anyone re-verifying), and the guard reaches --doctest-modules items too.

Test Plan

• uv run pytest -q — 2393 passed, 5 skipped, 11 xfailed (identical to baseline)
• uv run mypy / uv run ruff check nameparser/ clean
• uv run sphinx-build -b doctest docs — 177 tests, 0 failures
• Poison test: a stray warnings.warn in a test file fails the run

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