Fix for HVAC_Action HVAC_Mode Missmatch

[JackGlobetrotter]

Some thermostats (like _TZE200_hue3yfsn) will always report HVAC_Action as Heating regardless of the current state.
As a consequence state[ATTR_TEMPERATURE] is None which leads to the following error

2025-01-23 17:16:36.480 ERROR (Recorder) [homeassistant] Error doing job: Task exception was never retrieved (None)
Traceback (most recent call last):
  File "/config/custom_components/zoned_heating/switch.py", line 203, in async_zone_state_changed
    await self.async_calculate_override()
  File "/config/custom_components/zoned_heating/switch.py", line 209, in async_calculate_override
    await self.async_calculate_override_offset()
  File "/config/custom_components/zoned_heating/switch.py", line 262, in async_calculate_override_offset
    state[ATTR_TEMPERATURE] - state[ATTR_CURRENT_TEMPERATURE]

This PR addresses that issue by also checking HVAC_MODE which represents the real state (off when the thermostat is turned off, heat for heating).