[Bug]: [Update Required] SDK Location Inconsistency and Preview Model ID in intro_rag_engine.ipynb

### File Name

gemini/rag-engine/intro_rag_engine.ipynb

### What happened?

Inconsistency in SDK initialization and outdated Model ID.
Current Behavior:
The notebook initializes Vertex AI with location="us-east1" but sets the genai.Client to location="global". This mismatch causes authentication and resource discovery errors in the latest RAG Engine SDK updates.
The MODEL_ID is still using the gemini-3-flash-preview tag.

Expected Behavior:
Locations should be synchronized ( all set to us-central1) to ensure the RAG corpus and client can communicate properly.
The MODEL_ID should be updated to the stable gemini-3.1-flash since the stable version has been released (April 2026).

Steps to Reproduce:
Run the first two cells of the notebook.
Observe the mismatch in location parameters between vertexai.init and genai.Client.

vertexai.init(project=PROJECT_ID, location="us-east1")
client = genai.Client(vertexai=True, project=PROJECT_ID, location="global")

As a Data Science Researcher (MSc Informatics), I have already implemented these fixes in my fork and verified the stability. I am happy to open a Pull Request if access is granted.

### Relevant log output

```shell

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Open gemini/rag-engine/intro_rag_engine.ipynb and inspect the first two cells, where vertexai.init and genai.Client are configured. Verify that the location values are consistent and that MODEL_ID uses the requested stable model identifier, then rerun those cells to confirm initialization and resource discovery succeed.

Written by the indexing model from the issue text.