Add foundational analytics agent to GraphQL

What does this MR do and why?

Following the foundational agent docs, this MR adds the foundational analytics agent to GraphQL behind a feature flag.

• Adds a new FF
• Updates the resolver to add the agent with an FF
• Adds the agent to the agent definitions

Companion AI Gateway MR: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!3776 (merged)

References

• Move Duo Analytics Agent from AI Catalog to Fou... (#578342 - closed)

Screenshots or screen recordings

How to set up and validate locally

• Checkout changes from gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!3776 (merged) and !210291 (merged)
• Enable FF: foundational_analytics_agent
• Start a new agentic chat with Analytics Agent
• Try a query e.g. 'how many mrs were merged last week'

MR acceptance checklist

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

Related to #578342 (closed)