refactor: migrate from vertexai.preview to vertexai

[cnaples79]

Summary

Migrates RAG-related code from using the preview API to the stable API.

Changes Made

Updated import statements in RAG agent files.

Rationale

The Vertex AI RAG API has moved from preview to stable. Using the stable import ensures:

• Better API stability and support
• Alignment with current Vertex AI best practices
• Future compatibility as preview APIs may be deprecated

Testing

• Verified import statements are syntactically correct
• No functional changes to the code logic

Fixes #395