🔋 Fix: last_session_energy reported the full meter reading
On a real instance a ~3 h session reported 17.17 kWh — identical to the lifetime energy sensor — instead of the session delta.
Root cause: the per-session start snapshot was taken even when the cumulative energy value was still its 0.0 default (right after a restart, or when the integration updated mid-session before the API delivered the value). The end-of-session delta then became meter − 0 = the whole meter reading.
Fixes:
- The start snapshot is only taken once the cumulative energy is
> 0; otherwise no (misleading) delta is computed for that session - The delta is sanity-bounded to
[0, 50)kWh - The same bound applies on restore, so an implausible value can't survive restarts
No breaking changes, no option or translation changes.