## Feature Request

### What is your use case?

We are integrating with Microsoft Foundry Agent Service. We've deployed agents using the new Foundry Agents v2 experience, which uses the Responses API with `agent_reference` (referencing agents by name and version) rather than the deprecated Assistants API (thread/run pattern with `asst_*` IDs).

The current `azure_ai/agents/{agent_id}` route in LiteLLM implements the old Assistants API pattern, which: 1. Requires agent IDs beginning with `asst_` (but the new Foundry experience does not surface these IDs) 2. Uses the thread → message → run → poll flow (but the new Foundry Agents v2 uses `openai_client.responses.create()` with an `agent_reference` in `extra_body`)

Microsoft has [deprecated the Assistants API](https://learn.microsoft.com/en-us/azure/foundry-classic/openai/how-to/assistant) with retirement set for **August 26, 2026**.

### The new API pattern

From the Azure AI Foundry getting-started code:

```python from azure.ai.projects import AIProjectClient from azure.identity import DefaultAzureCredential

project_client = AIProjectClient( endpoint="https://<resource>.services.ai.azure.com/api/projects/<project>", creden…