Some earlier tests in the suite replace sys.modules['websockets'] with
a stub. When the smoke test ran after those, uvicorn's
websockets-sansio backend tried to import websockets.server at connect
time, resolved through the stub, and failed with
'No module named websockets.server; websockets is not a package',
preventing the mock proxy from starting.
Add an autouse fixture that drops stub websockets.* entries from
sys.modules, re-imports the real package + websockets.asyncio.server,
and restores prior state on teardown. Keeps the smoke test
deterministic regardless of collection order.
uvicorn's auto-select imports websockets_impl.py which requires the
legacy API (websockets<14). With websockets 16.0 installed in the
local venv, auto-select errored before its fallback could trigger,
failing the smoke test with 'No module named websockets.legacy'.
Explicitly selecting the websockets-sansio backend (shipped with
uvicorn) works against websockets 16.0 and keeps the smoke test
representative of the real production WS path.
