fix(story-2.2): _FailingTransport.stream annotation matches Transport protocol

ty rejected `def stream(...) -> None:` on _FailingTransport because the
Transport protocol requires the method to return
AbstractAsyncContextManager[StreamResponse]. Aligns with the
_OkTransport.stream stub shape used since Story 2-1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512

tests/test_middleware.py

@@ -322,7 +322,9 @@ def __init__(self, exc: BaseException) -> None:
     async def __call__(self, request: Request) -> Response:  # noqa: ARG002
         raise self._exc
 
-    def stream(self, request: Request) -> None:  # pragma: no cover - not exercised in 2-2
+    def stream(  # pragma: no cover - not exercised in 2-2
+        self, request: Request
+    ) -> AbstractAsyncContextManager[StreamResponse]:
         raise NotImplementedError
 
     async def aclose(self) -> None:  # pragma: no cover - not exercised in 2-2