fix(test): narrow type with isinstance to satisfy pyright

The test accesses DirectDispatcher internals (_in_flight_ids, _next_id)
but direct_pair() returns the Dispatcher protocol. Add an isinstance
assertion so pyright can see the concrete type.

Author: elang2

tests/shared/test_dispatcher.py

@@ -493,6 +493,7 @@ async def noop(
         return {}
 
     client, server, close = direct_pair()
+    assert isinstance(server, DirectDispatcher)
     try:
         async with anyio.create_task_group() as tg:
             await tg.start(client.run, noop, noop)