Add VertexAI support for summarizing pending comments
What does this MR do and why?
We want to move to VertexAI for summarizing review draft notes.
This is behind the summarize_review_vertex
feature flag.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
-
I have evaluated the MR acceptance checklist for this MR.
Related to #411928 (closed)