Add detection of pipelined requests in handle_client_read by checking if buffered data starts with a new HTTP request. When a pipelined request is detected, the server now sends a 400 Bad Request response with a descriptive body, closes the upstream connection, and transitions the client state to CLIENT_STATE_CLOSING. Previously, the server would silently close the upstream connection and continue reading headers. The change also includes a comprehensive test that validates the rejection behavior, response content, and connection state transitions for pipelined requests.