Docs: Document empty extras field on Duo Chat events in GraphQL API

Problem to solve

Customers querying AiUsageData.codeSuggestionEvents or directly querying ClickHouse ai_usage_events see that Duo Chat events (request_duo_chat_response, event_id 6) have extras: {} (empty). This causes confusion — customers assume data is missing or instrumentation is broken.

The empty extras is by design: Chat events don't have the language/suggestion metadata that Code Suggestions events carry. But this is not documented anywhere.

Further details

Affected docs:

• doc/api/graphql/duo_and_sdlc_trends.md — Event type documentation

Audience: Administrators and data engineers querying Duo analytics via GraphQL or direct ClickHouse access.

Proposal

Add a note to the API documentation:

Chat events (request_duo_chat_response) do not populate the extras field. Unlike Code Suggestions events, Chat interactions do not carry language or suggestion metadata. An empty extras object on Chat events is expected behavior.