Build the backtest WebSocket URL from a user-supplied endpoint: a bare
hostname becomes wss://{host}/backtest, while an explicit ws(s):// URL
keeps its scheme (for local development against plain-ws:// servers) and
gains the /backtest path only if missing. Any other scheme passes
through untouched. Idempotent, so already-normalized URLs survive the
crate::BacktestClient builder applying this to every url input.