Function subscribe_positions 

pub async fn subscribe_positions(
    client: &Arc<Client>,
    account_id: AccountId,
    position_tracker: PositionTracker,
    instrument_provider: Arc<InteractiveBrokersInstrumentProvider>,
) -> Result<()>

Subscribe to real-time position updates for detecting external position changes (e.g., option exercises).

This spawns a background task to track position changes and generate position status reports for external changes.

Errors

Returns an error if subscription fails.