Fix incorrect parameter name in AzureAIAgentClient initialization

[ry0y4n]

Summary

This PR fixes a bug in the tutorial example code where an incorrect parameter name was used when initializing AzureAIAgentClient.

Changes

• Changed async_credential=credential to credential=credential in the AzureAIAgentClient constructor call

Problem

The code was using async_credential as a parameter name, which does not exist in the AzureAIAgentClient API. This caused a ServiceInitializationError with the message:

agent_framework.exceptions.ServiceInitializationError: Azure credential is required when agents_client is not provided.

[learn-build-service-prod]

Learn Build status updates of commit c4fffa0:

✅ Validation status: passed

File	Status	Preview URL	Details
agent-framework/tutorials/agents/middleware.md	✅Succeeded

For more details, please refer to the build report.