chore: use billing event's unique event_id pass in Internal Events as well

What does this merge request do and why?

Related to https://gitlab.com/gitlab-org/architecture/usage-billing/design-doc/-/issues/10

Use billing event's unique event_id which is generated in AI Gateway to pass in Internal Events as well

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure Snowplow micro is setup and running to collect the events (doc).
3. Enable billing event in .env.

   AIGW_INTERNAL_EVENT__ENABLED=true

4. Send a query via Agentic Duo Chat or Flows in VSCode.
5. Confirm the usage_billing_event event is shown at http://0.0.0.0:9090/micro/good.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.