Make timestamp field required for journeyStepEventClass

Currently, the JourneyStepEvent class schema does not require @id and xdm:timestamp.

We propose making these fields required to ensure that:
- Any ingested data without a timestamp or id will be dropped during validation, preventing incomplete events from being processed.
- It is explicitly communicated to all consuming and producing teams that the timestamp and id fields are always expected to be populated.

This change does not enforce automatic timestamp or id generation, but it enforces data integrity and sets a clear contract for downstream consumers.

Requested Change:
- Add @id and xdm:timestamp andto the required array in journeyStepEventClass.schema.json.

Open journeyStepEventClass.schema.json and inspect its required array. Add @id and xdm:timestamp there, then use the repository's schema validation checks to confirm the schema remains valid. Done means both fields are required and data missing either field is rejected during validation.

Written by the indexing model from the issue text.