Migrate AiUsageData.codeSuggestionEvents endpoint to the new model

Summary

After data migrations are finished and new system "write" feature flag is enabled globally we should migrate codeSuggestionEvents endpoint to new data source i.e. Ai::UsageEvent model to create single source of truth for our data.

Proposal

Migrate AiUsageData.codeSuggestionEvents endpoint to new Ai::UsageEvent model.