Add return processing sample to openai_agents sample

[bachuv]

This pull request introduces a complete, orchestrated order return processing system using Azure Functions and OpenAI Agents. It implements a multi-step workflow for handling customer return requests, including validation, automated refund processing, and escalation to human review when necessary.

Order return validation:

• Adds order_return_validation.py defining an AI agent and business logic to validate return reasons against company policy, with structured result output.

Automated refund processing:

• Adds refund_processing.py defining an AI agent for processing refunds, including simulated payment operations and order detail retrieval.

Durable orchestrators and activities:

• Adds order_return_orchestrators.py registering orchestrators for the main order return process, validation, refund processing, and a human review activity.

Documentation and API reference:

• Adds ORDER_RETURNS.md with an overview, API reference, business logic, example requests/responses, architecture diagram, and testing instructions for the order return system.