Summary

Wires blockbuster into an
autouse fixture in tests/conftest.py so any synchronous IO that
slips into the asyncio event loop fails the test instead of going
unnoticed. Mirrors the integration just landed in
dbus-fast#690.

Downstream consumers (Home Assistant in particular) keep catching
async-loop blockers for us; surfacing them in this repo's own CI is
cheap and forces the fix at the source.

Details

• Adds blockbuster = ">=1.5.5,<2.0.0" to the dev dependency group.
• Adds a blockbuster autouse fixture that wraps every non-benchmark
  test in blockbuster_ctx(). Benchmarks under tests/benchmarks/
  are skipped — CodSpeed runs there should measure real workload, not
  pay the cost of monkeypatched builtins.
• Falls back to a no-op when blockbuster isn't importable (e.g. the
  PyPy / QEMU matrix legs that don't install it).
• pytest_collection_modifyitems marks entries in _KNOWN_BLOCKING
  as xfail(strict=False) so a future addition lands green and the
  list shrinks over time as the underlying blocks are removed.

The only test the full suite trips on today is
tests/test_core.py::Framework::test_use_asyncio_false_forces_thread_when_loop_running,
which is deliberately documenting the sync-bootstrap path that
Zeroconf(use_asyncio=False) takes from inside a running event loop
(it blocks waiting for the loop thread to come up — the test pins
that behavior). It stays xfail by design; the comment on
_KNOWN_BLOCKING calls that out.

Test plan

• `REQUIRE_CYTHON=1 poetry run pytest --timeout=60 tests` — 383 passed, 3 skipped, 2 xfailed (call + teardown of the by-design test above).
• `SKIP_CYTHON=1 poetry run pytest --timeout=60 tests` — same shape, confirms pure-Python path.
• `poetry run pre-commit run --files tests/conftest.py pyproject.toml poetry.lock` — clean.