Shadow the per-module runner lease in the docs subscriptions tests

The module parametrizes anyio_backend for its trio legs, which the
module-scoped lease cannot depend on (ScopeMismatch at setup); the lease's
own docstring prescribes this sync no-op shadow.

Author: maxisbey

tests/docs_src/test_subscriptions.py

@@ -30,6 +30,11 @@
 pytestmark = [pytest.mark.anyio, pytest.mark.filterwarnings("error::mcp.MCPDeprecationWarning")]
 
 
+@pytest.fixture(autouse=True)
+def _module_runner_lease() -> None:
+    """Opt out of the shared per-module event loop: this module parametrizes `anyio_backend`."""
+
+
 class _Stream:
     """Collects listen-stream notifications and lets tests await arrival counts."""