Data race between Start and Shutdown in SSEServer struct

There is data race between 

[func (s *SSEServer) Start(addr string) error](https://github.com/mark3labs/mcp-go/blob/37ac814a6010484d409bef15f6f4b015f486bdaa/server/sse.go#L192C2-L192C10)

and 

[func (s *SSEServer) Shutdown(ctx context.Context) error](https://github.com/mark3labs/mcp-go/blob/37ac814a6010484d409bef15f6f4b015f486bdaa/server/sse.go#L203C2-L203C4)
