fix(ocpp16): return Idle in TriggerMessage when not actively uploading/updating

fix(ocpp16): return Idle in TriggerMessage when not actively uploading/updating

Per spec §4.4/§7.24 and §4.5/§7.25, Idle SHALL only be sent via
TriggerMessage when not busy. The code was returning stale terminal
statuses (Uploaded, UploadFailed, Installed, etc.) instead of Idle.

- DiagnosticsStatusNotification: return Uploading only when actively
  uploading, Idle otherwise (fixes stale Uploaded/UploadFailed)
- FirmwareStatusNotification: return Downloading/Downloaded/Installing
  only when in progress, Idle otherwise (fixes stale Installed/Failed)
- UpdateFirmware guard: allow retry after DownloadFailed or
  InstallationFailed instead of blocking all non-Installed statuses