Fix NuosSplit operative mode changes (GREEN mode not working)

[eye0fra]

Problem

Setting the operative mode on NuosSplit devices (e.g., Nuos Plus S2) via set_nuos_mode() / async_set_nuos_mode() returned {"success": true} but the mode never actually changed.

Root Cause

The Ariston API requires the "old" field in the request payload to apply operative mode changes. The library was only sending {"new": value}.

Fix

• Updated set_nuos_mode() and async_set_nuos_mode() in ariston_api.py to accept and include the old_value parameter
• Updated set_water_heater_operation_mode() and async_set_water_heater_operation_mode() in nuos_split_device.py to pass the current mode value as old

Before

{"new": 0}  # Accepted but ignored

After

{"new": 0, "old": 1}  # Actually applies the change

Related to fustom/ariston-remotethermo-home-assistant-v3#407