refactor(ocpp): consolidate MeterValues validators and helpers
Replace 5 near-identical validators with 1 generic
validateMeasurandValue using options pattern for phase
and interval. Merge addLineToLineVoltageToMeterValue into
addPhaseVoltageToMeterValue with nominalVoltage and optional
noTemplateFallback params preserving voltage simulation
semantics. Inline dead getMeasurandDefaultContext and
getMeasurandDefault. Extract magic numbers as constants.