AssertionError: Unexpected message while using middlewares

Describe the bug
when I mount the mcp to starlette, and I use middleware to do some prechecks, I met this errors.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/app/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
result = await app( # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/app/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in call
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/app/.venv/lib/python3.12/site-packages/starlette/applications.py", line 113, in call
await self.middleware_stack(scope, receive, send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in call
raise exc
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in call
await self.app(scope, receive, _send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 182, in call
with recv_stream, send_stream, collapse_excgroups():
^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/contextlib.py", line 158, in exit
self.gen.throw(value)
File "/app/.venv/lib/python3.12/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
raise exc
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 185, in call
await response(scope, wrapped_receive, send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 223, in call
async for chunk in self.body_iterator:
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 169, in body_stream
assert message["type"] == "http.response.body", f"Unexpected message: {message}"
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'0')]}

To Reproduce
Steps to reproduce the behavior:

1. mount mcp to starlette
2. use middlewares
3. call tools

class CustomHeaderMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        return await call_next(request)


middleware = [
    Middleware(CustomHeaderMiddleware)
]

app = Starlette(middleware=middleware)
app.mount("/", mcp_server.sse_app())

def main():
    uvicorn.run(
        app="atlantis_mcp_server:app",
        host="0.0.0.0",
        port=8000,
        workers=1,
        log_level="info"
    )

__all__ = ["main"]

Expected behavior
Should not raise type error

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

Comienza con el montaje de la aplicación de Starlette mcp_server.sse_app(), el ejemplo de CustomHeaderMiddleware y la ruta de llamada del middleware mostrada en el traceback. Reproduce el fallo llamando a una herramienta a través de la aplicación SSE montada y, después, sigue los mensajes de respuesta hasta entender el mensaje inesperado. El trabajo estará terminado cuando la aplicación envuelta por el middleware complete la solicitud sin AssertionError.