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

### What happened?

`/v1/responses/compact` fails during `router_settings.fallbacks` failover from a primary model to its fallback, with Azure returning `400 Unknown parameter: 'metadata'`. The same failover on `/v1/responses` (non-compact) works correctly and returns 200 from the fallback deployment.

Root cause is in `litellm/router_utils/fallback_event_handlers.py::run_async_fallback` at line 133 (currently identical between tag `v1.82.3` and tip of `main`):

```python kwargs.setdefault("metadata", {}).update( {"model_group": kwargs.get("model", None)} ) # update model_group used, if fallbacks are done ```

This unconditionally writes to the `"metadata"` key in kwargs. For Responses API routes (paths containing `"responses"` per `LITELLM_METADATA_ROUTES` in `litellm/proxy/litellm_pre_call_utils.py:65-70`), the proxy's correct metadata-variable name is `"litellm_metadata"`, not `"metadata"`. The fallback path writes a top-level `"metadata"` dict into kwargs that then gets forwarded as a request-body parameter to the upstream provider. Azure OpenAI's v1-compat `/responses/compact` endpoin…