Rename new endpoint from ai/events_collector to ai/billable_usage/events

What does this MR do and why?

Summary

This change renames an API endpoint used for collecting AI-related usage events. The old endpoint path (/ai/events_collector/com.snowplowanalytics.snowplow/tp2) is being replaced with a new one (/ai/billable_usage/events/com.snowplowanalytics.snowplow/tp2). The snapshot test file is updated to reflect this rename, removing references to the old path and adding the new one. This is essentially a renaming/restructuring of a tracking endpoint, likely to better reflect its purpose of capturing billable AI usage data.

Related to gitlab-org/gitlab!259290 (merged)

See this discussion for more context.

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Closes #631688