Instrument natural language querying feature usage

What does this MR do and why?

Adds user event tracking for the GitLab Duo natural language querying feature, within the product analytics visualization designer.

One new internal event is added - tracked from the frontend - user_submitted_gitlab_duo_query_from_visualization_designer

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Apply setup steps for backend MR this one depends on:

• Enable AI features locally: https://docs.gitlab.com/ee/development/ai_features/#test-ai-features-locally
• Make sure you've enabled Vertex access and configured it with your GDK. This action does not use Anthropic's API, only Vertex.
• Enable the :generate_cube_query flag locally. NEW

Then:

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Type in a prompt and click "Generate with Duo"
5. Verify user_submitted_gitlab_duo_query_from_visualization_designer tracking event is submitted

Related to #442052 (closed)