fix: propagate stream_item_type when using include_router

Fixes #15401: SSE stream_item_type was not copied from source routes to
merged routes when using APIRouter + include_router. The stream detection
branch in APIRoute.__init__ only runs when response_model is a
DefaultPlaceholder. After include_router copies route.response_model (which
is already None), the branch is skipped and stream_item_type stays None.

Now explicitly copy stream_item_type and stream_item_field from source route
to the merged route after add_api_route creates it.

fastapi/routing.py

@@ -1794,6 +1794,11 @@ class APIRouter(routing.Router):
                         self.strict_content_type,
                     ),
                 )
+                if route.stream_item_type is not None:
+                    new_route = self.routes[-1]
+                    assert isinstance(new_route, APIRoute)
+                    new_route.stream_item_type = route.stream_item_type
+                    new_route.stream_item_field = route.stream_item_field
             elif isinstance(route, routing.Route):
                 methods = list(route.methods or [])
                 self.add_route(