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/templates/starter/run.yaml

@@ -26,7 +26,7 @@ providers:
   - provider_id: ${env.ENABLE_VLLM:=__disabled__}
     provider_type: remote::vllm
     config:
-      url: ${env.VLLM_URL}
+      url: ${env.VLLM_URL:=}
       max_tokens: ${env.VLLM_MAX_TOKENS:=4096}
       api_token: ${env.VLLM_API_TOKEN:=fake}
       tls_verify: ${env.VLLM_TLS_VERIFY:=true}