mount_path does not work with sse_app

[kinch-tech]

while i start mcp server with mcp.run, provided with mount_path, then client get bad response with 404。

the endpoint for handling post message should be normalized_message_endpoint, not the original message_path

        routes.append(
            Mount(
                self.settings.message_path,
                app=sse.handle_post_message,
            )
        )

[adamsaimi]

Hello i opened a merge request, there was no 1.9.x branch available so I targeted main.