fix(ocpp20): isolate VariableManager overrides per station

fix(ocpp20): isolate VariableManager overrides per station

runtimeOverrides, minSetOverrides, and maxSetOverrides were stored as flat
Maps on the singleton OCPP20VariableManager, causing SetVariables on one
station to affect all others sharing the same service instance.

Apply the same Map<stationId, Map<key, value>> pattern already used by
invalidVariables in the same class. resetRuntimeOverrides now accepts
optional stationId, matching the invalidateMappingsCache API.