openai-agents-python v0.18.0 sets gpt-realtime-2.1 as default for RealtimeAgents

The openai-agents-python library has released version 0.18.0, which updates the default model used by RealtimeAgents to gpt-realtime-2.1.

• Updates the default realtime model to gpt-realtime-2.1.
• Adds a Unicode storage option to SQLAlchemySession.
• Fixes visualization issues by preserving fillcolor on handoff nodes through merged style attributes.