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.

Bắt đầu với việc mount ứng dụng Starlette mcp_server.sse_app(), ví dụ CustomHeaderMiddleware và đường dẫn gọi middleware được hiển thị trong traceback. Tái hiện lỗi bằng cách gọi một tool thông qua ứng dụng SSE đã được mount, sau đó theo dõi các message phản hồi cho đến khi hiểu được message bất ngờ. Hoàn thành khi ứng dụng được middleware bọc quanh xử lý xong request mà không gặp AssertionError.