refactor(ocpp16): track diagnosticsStatus and extract composite schedule helper

refactor(ocpp16): track diagnosticsStatus and extract composite schedule helper

- Add diagnosticsStatus to ChargingStationInfo, set at each
  DiagnosticsStatusNotification send point (Uploading, Uploaded,
  UploadFailed). TriggerMessage now reports actual diagnostics status
  instead of hardcoded Idle, mirroring the firmwareStatus pattern.

- Extract composeCompositeSchedule() private method, eliminating
  ~120 lines of duplicated profile preparation logic between the
  connectorId=0 and connectorId>0 branches of GetCompositeSchedule.