Marketing Cloud ID is used in EndUserIDs instead of Experience Cloud ID

We should refer to this ID experience cloud and use that as the key, it's not consistent with the code that is used for ECID fo `ECID`.
https://github.com/adobe/xdm/blob/035d53be03861efc152cca620237751cc5ce551c/schemas/context/enduserids.schema.json#L19

Open schemas/context/enduserids.schema.json at the linked line and inspect the EndUserIDs key alongside the ECID naming used in the schema. Update the identifier naming to refer to the Experience Cloud ID consistently, then validate the schema or its existing checks if available.

Written by the indexing model from the issue text.