Add hvac_action property to climate entity

[lnagel]

Implement hvac_action property that reports the current operating state
of the device using native Modbus status register (REG_STATUS) bitmasks.

Maps HVAC actions based on priority:

• OFF: Device power is off (REG_POWER = 0)
• HEATING: Heating bit is set (BITMASK_HEATING)
• COOLING: Cooling bit is set (BITMASK_COOLING)
• FAN: Fan bit is set (BITMASK_FAN)
• IDLE: Device is on but no active operations

The implementation reuses existing bitmask constants from binary_sensor.py
and requires no additional Modbus reads since REG_STATUS is already
fetched by the coordinator.

Tests cover all action states, priority handling, and edge cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (882bcdf) to head (261bc10).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
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+ Coverage   90.33%   90.46%   +0.13%     
==========================================
  Files          18       18              
  Lines        1314     1332      +18     
  Branches      127      133       +6     
==========================================
+ Hits         1187     1205      +18     
  Misses         95       95              
  Partials       32       32              

Files with missing lines	Coverage Δ
custom_components/komfovent/climate.py	94.82% <100.00%> (+0.95%)	⬆️