fix: replace assert with raise FastAPIError in routing.py

Three `assert isinstance(...)` guards in routing.py are used as runtime
checks for required ASGI scope keys (fastapi_middleware_astack,
fastapi_inner_astack). Under `python -O`, all assert statements are
stripped, converting these diagnostic guards into silent AttributeError
exceptions with no context.

Replace with explicit `if not isinstance(...): raise FastAPIError(...)`
to preserve the error messages regardless of Python optimization level.

Affects:
- get_request_handler() HTTP middleware stack check (line 349)
- get_request_handler() HTTP dependency stack check (line 418)
- get_websocket_app() WebSocket dependency stack check (line 514)

fastapi/routing.py

@@ -346,7 +346,8 @@ def get_request_handler(
     async def app(request: Request) -> Response:
         response: Response | None = None
         file_stack = request.scope.get("fastapi_middleware_astack")
-        assert isinstance(file_stack, AsyncExitStack), (
+        if not isinstance(file_stack, AsyncExitStack):
+            raise FastAPIError(
                 "fastapi_middleware_astack not found in request scope"
             )
 
@@ -415,7 +416,8 @@ def get_request_handler(
         # Solve dependencies and run path operation function, auto-closing dependencies
         errors: list[Any] = []
         async_exit_stack = request.scope.get("fastapi_inner_astack")
-        assert isinstance(async_exit_stack, AsyncExitStack), (
+        if not isinstance(async_exit_stack, AsyncExitStack):
+            raise FastAPIError(
                 "fastapi_inner_astack not found in request scope"
             )
         solved_result = await solve_dependencies(
@@ -511,7 +513,8 @@ def get_websocket_app(
             mount_path = websocket.scope.get("root_path", "").rstrip("/")
             endpoint_ctx["path"] = f"WS {mount_path}{dependant.path}"
         async_exit_stack = websocket.scope.get("fastapi_inner_astack")
-        assert isinstance(async_exit_stack, AsyncExitStack), (
+        if not isinstance(async_exit_stack, AsyncExitStack):
+            raise FastAPIError(
                 "fastapi_inner_astack not found in request scope"
             )
         solved_result = await solve_dependencies(