chore: update the vLLM inference impl to use OpenAIMixin for openai-compat functions

inference recordings from Qwen3-0.6B and vLLM 0.8.3 -
```
docker run --gpus all -v ~/.cache/huggingface:/root/.cache/huggingface -p 8000:8000 --ipc=host \
    vllm/vllm-openai:latest \
    --model Qwen/Qwen3-0.6B --enable-auto-tool-choice --tool-call-parser hermes
```

test with -

```
./scripts/integration-tests.sh --stack-config server:ci-tests --setup vllm --subdirs inference
```

tests/integration/suites.py

@@ -78,7 +78,7 @@ SETUP_DEFINITIONS: dict[str, Setup] = {
             "VLLM_URL": "http://localhost:8000/v1",
         },
         defaults={
-            "text_model": "vllm/meta-llama/Llama-3.2-1B-Instruct",
+            "text_model": "vllm/Qwen/Qwen3-0.6B",
             "embedding_model": "sentence-transformers/all-MiniLM-L6-v2",
         },
     ),