Oh I see, you want to automate your trading system as well, not just backtest it.
You can probably send automated, one-way trading signals to your broker (from your C# code for example) via its API.
Not necessarily automate the trading part, just the alerts. I can place the trades manually with my broker, but need the strategy to run intraday and alert me to the buy signal at the right time. So broker API not needed.
Back to the original question though - whether backtesting or running intraday, is there a platform that is trustworthy and (at least) claims upfront that they don't upload any of your custom code?
E.g. Ninjatrader privacy policy explicitly states that they DO NOT guarantee any such protection. Appalling!