Refresh extra battery serial number and model in the device registry

[GnoX]

The serial number of an extra battery rides in only about half of the device's DisplayPropertyUpload messages, so it can arrive after the extra-battery entity - and its device registry entry - were first created. EcoflowBatteryAddonEntity.device_info exposes serial_number (and the model name derived from it via battery_name_from_device), but HA reads device_info only once when the entity is added. If no SN-bearing message had landed before platform setup, the registry entry was written with serial_number=None and a fallback "Extra Battery" model and then never refreshed - so the serial number was missing "in some cases" depending on timing.

This PR makes EcoflowBatteryAddonEntity register a state-update callback on battery_{index}_sn and pushes the value into the device registry (async_update_device) once it is known, updating both the serial number and the model. It is a no-op when the SN is empty, the registry entry is missing, or the entry is already current.

Should address discussion in #362

[rabits]

Looks good, thank you!