Fix: remove hardcoded us-central1 for RAG engine region

[oxlipefe]

Description

This PR removes the hardcoded us-central1 location in the RAG example and instead reads the region from the GOOGLE_CLOUD_LOCATION environment variable.

Motivation

For new Google Cloud projects, RAG Engine is restricted in us-central1, us-east1, and us-east4. The current example fails with an INVALID_ARGUMENT error in those regions.

Using the environment variable improves flexibility and prevents unnecessary errors.

Changes

• Replaced hardcoded region in - adk-samples/python/agents/data-science/utils/reference_guide_RAG.py - with environment variable fallback
• Preserved backward compatibility by defaulting to us-central1

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