Implement TES temperature boosting as constraints

[TomKae00]

Describe the feature you'd like to see

In PR #1711, we added PTES temperature boosting via resistive heaters and air-sourced heat pumps directly in prepare_sector_network.py. After discussing with @amos-schledorn, we’ve decided to move this logic into solve_sector_network.py as explicit constraints. This refactor keeps prepare_sector_network.py clear and makes it easier to generalize boosting for other TES: TTES, ATES

The config forwarding fix for heat_pump_cop_approximation is handled separately in PR #1729 .