fix(coordinator): 🐛 stop hiding heating/cooling devices before first use

[rhammen]

🐛 Problem

Reported in #655: on a freshly commissioned system, the heating and cooling devices (and every entity under them) were missing entirely, even though DHW showed up fine.

Cause: has_heating/has_cooling gated the entire device on cumulative operating hours reported by the heat pump (> 0). If a mode simply hadn't run yet — e.g. heating/cooling never activated on a newly poured floor, or cooling checked outside the season — its device was hidden instead of just showing zeroed-out sensors. DHW only worked for the reporter by coincidence, since it had already accumulated hours.

✅ Fix

• Heating: now always creates the device, the same way the heatpump device is always created. Every supported Luxtronik heat pump can heat, so there isn't a meaningful "not present" case here — removed the now-dead has_heating property.
• Cooling: creates the device if usage hours are > 0 or detect_cooling_present() says a mixing circuit is configured for cooling/heating_cooling. This reuses the existing, already-trusted MK-based detection (previously only used for per-entity visibility of V0005_COOLING), so cooling shows up as soon as it's configured rather than after it's actually run once.
• Domestic water is intentionally left unchanged — DHW is used year-round (not seasonal like heating/cooling), so a system with 0 DHW hours self-corrects within days. We'll only revisit it if it turns out to be a real problem in practice.

🧪 Test plan

• Added test_heating_always_active_without_usage_hours (heating device active even with 0 hours)
• Added test_cooling_active_without_usage_hours_when_configured (cooling device active with 0 hours but MK1 configured as cooling)
• pytest — 747 passed, 100% coverage maintained
• ruff check / ruff format --check — clean
• basedpyright — 0 errors

Resolves #655