feat: add bidirectional temperature conversion with Fahrenheit API

[eman]

Summary

This PR simplifies the temperature API by allowing users to pass temperatures in Fahrenheit directly, with automatic conversion to the device's internal format (half-degrees Celsius).

Breaking Changes

• build_reservation_entry(): Now accepts temperature_f (Fahrenheit) instead of raw param value
• set_dhw_temperature(): Now accepts temperature_f: float instead of raw integer
• Removed set_dhw_temperature_display(): This method used an incorrect conversion formula

Changes

Added

• fahrenheit_to_half_celsius() utility function exported from main package

Fixed

• Temperature encoding bug in set_dhw_temperature() that used incorrect 'subtract 20' conversion instead of proper half-degrees Celsius encoding

Migration Guide

# Reservations - OLD
build_reservation_entry(..., param=120)

# Reservations - NEW  
build_reservation_entry(..., temperature_f=140.0)

# DHW Temperature - OLD
await mqtt.set_dhw_temperature(device, 120)
await mqtt.set_dhw_temperature_display(device, 140)

# DHW Temperature - NEW
await mqtt.set_dhw_temperature(device, 140.0)

Validation

• ✅ Linting: All checks passed
• ✅ Type checking: No errors
• ✅ Tests: 127 passed

[Copilot]

Pull request overview

This PR simplifies temperature handling by allowing direct Fahrenheit input across the API, with automatic conversion to the device's internal half-degrees Celsius format. The key improvement is fixing an incorrect "subtract 20" conversion formula that was previously used.

Key Changes:

• Replaced param with temperature_f in build_reservation_entry() for clearer API
• Fixed temperature encoding bug in set_dhw_temperature() that used incorrect conversion
• Removed set_dhw_temperature_display() method (used incorrect formula)

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/nwp500/models.py	Added fahrenheit_to_half_celsius() utility function for proper temperature conversion
src/nwp500/encoding.py	Updated build_reservation_entry() to accept temperature_f parameter with validation
src/nwp500/mqtt_device_control.py	Fixed set_dhw_temperature() to use proper conversion, removed incorrect display method
src/nwp500/mqtt_client.py	Updated public API to match device controller changes
src/nwp500/cli/commands.py	Updated CLI to use corrected temperature range and method
src/nwp500/cli/main.py	Changed argument type from int to float
src/nwp500/init.py	Exported new utility function
tests/test_models.py	Added comprehensive tests for conversion function
tests/test_api_helpers.py	Updated tests to use new API and validate conversions
examples/reservation_schedule_example.py	Updated example to use new temperature parameter
docs/*	Updated documentation throughout to reflect API changes
CHANGELOG.rst	Documented breaking changes and migration guide