Add agent platform event counts to GraphQL

What does this MR do and why?

This merge request enhances the AI metrics system by adding detailed session tracking for agent platform events. The changes introduce new fields to track different session states (created, started, finished, dropped, stopped, resumed) and add filtering capabilities to query these metrics by flow type.

The implementation includes a new service to count these session events from a ClickHouse database, updates the GraphQL API to expose these new metrics with filtering options, and refactors some shared code to be more reusable across different AI analytics features. The changes also include comprehensive tests to ensure the new functionality works correctly with various filtering scenarios.

References

• #570563

How to set up and validate locally

1. Run FILTER=ai_usage_stats bundle exec rake db:seed_fu, try lowering TIME_PERIOD_DAYS to 5 to have more consistent data.

2. Test the query with the flow type inclusion filter

query {
  group(fullPath: "toolbox") {
    name
    
    aiMetrics {
      agentPlatform(flowTypes: ["duo_chat"]) {
        createdSessionEventCount
        startedSessionEventCount
        finishedSessionEventCount
        stoppedSessionEventCount
        resumedSessionEventCount
        droppedSessionEventCount
      }
    }
  }

3 Test the query with the flow type exclusion filter

query {
  group(fullPath: "toolbox") {
    name
    
    aiMetrics {
      agentPlatform(not: { flowTypes: ["duo_chat"] }) {
        createdSessionEventCount
        startedSessionEventCount
        finishedSessionEventCount
        stoppedSessionEventCount
        resumedSessionEventCount
        droppedSessionEventCount
      }
    }
  }

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