Update OpenAI model in email assistant example

[goutamk09]

Partially implements #521

This PR updates the email assistant example to replace deprecated OpenAI
model usage with a current recommended model.

Changes

• Updated examples/email-assistant/application.py
• Replaced gpt-4 with gpt-4o-mini

How I tested this

• Verified the example runs without runtime errors locally

Notes

• Example-only change
• No core Burr logic modified

[goutamk09]

Hi @skrawcz, Thanks for the review!

I tested the email assistant example locally after updating the OpenAI model.

Steps run:

• pip install -r requirements.txt
• python application.py

The application initializes successfully and proceeds until Graphviz rendering.
The Graphviz dot error is expected on Windows without system Graphviz installed
and is unrelated to the OpenAI model change.

Attaching screenshots below for validation.