### What happened?

`POST /fallback` accepts any model name in the request body, including provider-prefixed names with slashes (`openrouter/gpt-4`). The read and delete counterparts declare the name as a path parameter with Starlette's default `str` converter, which matches a single path segment only:

```python @router.get("/fallback/{model}", ...) @router.delete("/fallback/{model}", ...) ```

A model name containing a slash can never match these routes. URL-encoding does not help because the ASGI server decodes `%2F` back to `/` before routing, so the request never reaches the handler and returns Starlette's default `404 {"detail":"Not Found"}`

Reproduction, config:

```yaml model_list: - model_name: openrouter/gpt-4 litellm_params: model: openai/gpt-4 api_key: sk-fake - model_name: gpt-4-backup litellm_params: model: openai/gpt-4 api_key: sk-fake

router_settings: fallbacks: - "openrouter/gpt-4": ["gpt-4-backup"]

general_settings: master_key: sk-1234 ```

```bash $ curl -s -w "\nHTTP %{http_code}\n" "http://localhost:4000/fallback/openrouter%2Fgpt-4?fallback_type=general" -H "Authorization: Bearer sk-1234" {"detail":"Not Found"} HTT…