### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

When the LiteLLM router falls back from one deployment to another (e.g., `gpt-4o` on Azure East ? `gpt-4o` on Azure West, or `gpt-4o` ? `gpt-4-turbo`), the `model` field in the response and the `x-litellm-model` response header still reflect the originally requested model group name, not the deployment that actually served the request.

Proposal: add a field to the response (or response header) that identifies the actual deployment/model that served the request after fallback resolution.

Suggested additions: - `x-litellm-actual-model` response header: the `litellm_params.model` of the deployment that served the call - `x-litellm-model-group` response header: the requested model group name (existing `x-litellm-model` behavior, renamed for clarity) - Alternatively, populate `usage.model` or a `_hidden_params.model_used` field with the resolved model

### Motivation, pitch

Without knowing which deployment actually served a request:

1. **Cost attribution breaks**: if the fallback lands on a more expensive model (e.g., GPT-4 instead of GPT-4o-mini),…