fix(tests): drop illegal @pytest.mark.anyio on anyio_backend fixture

Newer pytest raises 'Marks cannot be applied to fixtures' instead of ignoring
it. The async test methods already carry @pytest.mark.anyio, which is what
selects the backend; the fixture must not.

tests/fleet/test_reconciler.py

@@ -142,7 +142,6 @@ class TestAggregate:
 
 # ── reconcile_once ────────────────────────────────────────────────────────────
 
-@pytest.mark.anyio
 @pytest.fixture
 def anyio_backend():
     return "asyncio"