Update OpenAI model in conversational RAG example

[goutamk09]

Partially implements #521

This PR updates one of the example applications to replace deprecated OpenAI
model usage with the latest recommended model.

Changes

• Updated examples/conversational-rag/graph_db_example/application.py
• Replaced deprecated model (gpt-4-turbo-preview) with gpt-4o-mini

How I tested this

• Verified the example code updates cleanly
• No changes made to core Burr logic

Notes

• This is an incremental update; happy to continue updating additional examples
  once this approach is confirmed.

[skrawcz]

thanks! looks good. Can you grep the code base for model="gpt and update those examples and open more PRs please. There might be some prompt tuning needed, but hopefully not. Screenshots of testing would be great validation as well.