Add sse_app method and update run_sse_async method in server.py

* Add `sse_app` method to return an instance of the SSE server app
* Update `run_sse_async` method to use the new `sse_app` method

Update `README.md` to include instructions for mounting SSE server

* Add section on mounting the SSE server to an existing ASGI server

Author: comfuture

src/mcp/server/fastmcp/server.py

@@ -461,9 +461,6 @@ async def run_stdio_async(self) -> None:
 
     async def run_sse_async(self) -> None:
         """Run the server using SSE transport."""
-        from starlette.applications import Starlette
-        from starlette.routing import Mount, Route
-
         starlette_app = self.sse_app()
 
         config = uvicorn.Config(
@@ -477,9 +474,6 @@ async def run_sse_async(self) -> None:
 
     def sse_app(self) -> Starlette:
         """Return an instance of the SSE server app."""
-        from starlette.applications import Starlette
-        from starlette.routing import Mount, Route
-
         sse = SseServerTransport("/messages/")
 
         async def handle_sse(request):