fix(example): use openai:gpt-4o-mini to avoid CI failure due to missing CO_API_KEY

Author: sijanonly

examples/pydantic_ai_examples/medical_agent_delegation.py

@@ -33,8 +33,7 @@
 
 from pydantic_ai import Agent, ModelHTTPError, RunContext
 
-# Make sure to set CO_API_KEY token. Or Change the model and token based on relevant provider
-MODEL = 'cohere:command-r7b-12-2024'
+MODEL = 'openai:gpt-4.1-mini'
 
 
 # Structured Outputs