Add AiTracking for Duo Chat REST API path

What does this MR do and why?

Adds track_internal_event('request_duo_chat_response', ...) to Gitlab::Duo::Chat::Completions#execute so IDE-originated Duo Chat via POST /api/v4/chat/completions lands in ai_usage_events.

The Web UI path (Llm::ChatService#perform) already fires this event, and Gitlab::Tracking::AiTracking is now wired automatically as a default tracker via ee/lib/ee/gitlab/tracking/event_definition.rb, so no event YAML change is needed. The IDE REST endpoint calls Completions#execute directly rather than through ChatService, so it was the one path still missing the tracking call.

This closes the IDE Duo Chat row of the audit at #567159. Without it, IDE Chat is invisible to AiUsageData, AiUserMetrics, and the Duo and SDLC Trends dashboard.

Tracks #567159.

How to set up and validate locally

1. Stand up a self-managed GitLab instance with ClickHouse configured.
2. Run the new spec:

   bundle exec rspec ee/spec/lib/gitlab/duo/chat/completions_spec.rb -e "#execute attribution"

3. To verify end-to-end: invoke Gitlab::Duo::Chat::Completions.new(user, organization: org).execute(safe_params: { content: 'hi' }) in a rails console. After the write buffer flushes, Ai::UsageEvent.where(event: :request_duo_chat_response).count increases by 1, attributed to the invoking user.

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