Apply half-spread and commission on both open/close trade

### Enhancement description

Hi,

Full spread is currently applied to buy price on trade open and open+close commission only on trade close. This doesn't reflect real life and affects backtesting stop loss/trailing stop loss trigger level.
Also,, I'm using trade.pl_pct for trailing stop loss calculation and I have to subtract commission manually.
It also leads to bad optimization.

The situation should be :
- trade open: apply half-spread on buy price and also apply commission
- trade close: apply half-spread on sell price and also apply commission

Thanks a lot!

### Code sample

```python

```

### Additional info, images

_No response_

Start by locating the trade-open and trade-close price and commission calculations in the backtesting engine, then trace how they affect stop-loss, trailing-stop-loss, and trade.pl_pct values. Done means each side applies half-spread and commission, with backtests confirming the corrected trigger levels and optimization behavior.

Written by the indexing model from the issue text.