Support proper N/A display for device features that report 0 to indicate
'not available' rather than actual zero readings. This addresses confusion
where missing sensors showed misleading values like 0.0°C/32.0°F instead of N/A.

Three categories of N/A support:
- Temperature sensors (outside, inlet, recirculation, etc.) - when sensor doesn't exist
- Optional feature settings (mixing valve, heating element lower) - when feature not present or not applicable to current mode
- Recirculation pump status - when no pump installed

Key changes:
- Modified 4 temperature converters to return float | None (return None when value is 0)
- Created new converters for optional feature values (int_with_zero_as_none, float_with_zero_as_none, etc.)
- Updated ~25 fields in DeviceStatus model to support optional values
- CLI now displays 'N/A' instead of misleading zero values
- Added comprehensive test coverage (6 new tests)
- Documented all changes in CHANGES.md

- Use ternary operator for N/A value formatting (SIM108)
- Shorten test docstrings to meet line length limit (E501)
- Add None check for recirc_operation_mode before accessing attributes
  to fix type errors in handlers.py (reportOptionalMemberAccess)

Changed '# Should be None' to '# 0 will become None' to accurately
describe that the input value of 0 will be converted to None during
validation, addressing review feedback from Copilot.

BREAKING CHANGE: Heat pump sensors now correctly report 0°C/32°F instead
of None for freezing temperatures.

Problem:
The previous implementation treated 0 as a universal sentinel for "N/A",
causing heat pump compressor/refrigerant sensors to incorrectly report
"unavailable" when measuring 0°C during normal operation.

Solution:
- Removed zero-as-none from deci_celsius_to_preferred() converter
  (used for heat pump sensors with 0.1°C precision)
- Kept zero-as-none for half_celsius_to_preferred() converter
  (used for optional sensors and mode-dependent settings)
- Kept zero-as-none for raw_celsius_to_preferred() converter
  (used for optional outside sensor)

Protocol Analysis:
The device protocol uses different temperature precision for different
sensor types with different semantics:
- DeciCelsius (0.1°C): Heat pump sensors - 0 = valid 0°C reading
- HalfCelsius (0.5°C): Optional/mode-dependent - 0 = N/A
- RawCelsius: Optional external sensor - 0 = not installed

Type Safety:
- Added None-handling to _format_number() in CLI formatters
- Updated example code to safely format optional temperatures
- All tests passing (410 passed, 3 skipped)

Fixes issue identified in reference/na_evaluation.txt