Change Duo Chat documentation search endpoint

What does this MR do and why?

Update API endpoint. This code path is currently behind the ai_gateway_docs_search feature flag.

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

How to set up and validate locally

1. In Rails conosle: Feature.enable(:ai_gateway_docs_search)
2. In AI Gateway, check out vertex-ai-search branch.
3. Ask a documentation related question in Duo Chat and see answers.

Related to #451215 (closed)