[Feature] Connector Change

### Is there an existing feature request for this?

- [x] I have searched the existing issues

### Describe the Feature

Add `batch_event_index` to the `event_id` surrogate key. This is in response to new fields that were added to our GA4 Export Connector, which includes the aforementioned.

### How would you implement this feature?

This would be a change in the staging event model, which will then flow downstream

### Describe alternatives you've considered

_No response_

### Are you interested in contributing this feature?

- [x] Yes.
- [ ] Yes, but I will need assistance.
- [ ] No.

### Anything else?

_No response_

Start by locating the staging event model referenced in the issue and trace how its surrogate event_id key flows downstream. Update the key to include batch_event_index, then verify that the downstream models continue to receive the expected event identifiers and new GA4 Export Connector fields.

Written by the indexing model from the issue text.