Fix #1335: Apply half-spread and commission symmetrically on open and close

[dwaddle]

Fixes #1335

### Enhancement description
This PR fixes the asymmetry where the full spread was applied to the buy price on trade open (and 0 on close), and where commissions were only subtracted after the

trade was closed.

Changes applied:
- `_adjusted_price` now applies half of the spread on trade open.
- `_close_trade` now applies the opposite half of the spread on the exit price.
- `trade._commissions` is immediately populated on trade open, ensuring `trade.pl` and `trade.pl_pct` reflect the correct Mark-to-Market value during the trade's

lifetime (which is crucial for trailing stop losses).