feat(vllm): periodically refresh models (#2823)

Just like #2805 but for vLLM.

We also make VLLM_URL env variable optional (not required) -- if not
specified, the provider silently sits idle and yells eventually if
someone tries to call a completion on it. This is done so as to allow
this provider to be present in the `starter` distribution.

## Test Plan

Set up vLLM, copy the starter template and set `{ refresh_models: true,
refresh_models_interval: 10 }` for the vllm provider and then run:

```
ENABLE_VLLM=vllm VLLM_URL=http://localhost:8000/v1 \
  uv run llama stack run --image-type venv /tmp/starter.yaml
```

Verify that `llama-stack-client models list` brings up the model
correctly from vLLM.

llama_stack/apis/inference/inference.py

@@ -819,7 +819,7 @@ class OpenAIEmbeddingsResponse(BaseModel):
 class ModelStore(Protocol):
     async def get_model(self, identifier: str) -> Model: ...
 
-    async def update_registered_models(
+    async def update_registered_llm_models(
         self,
         provider_id: str,
         models: list[Model],