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/recordings/responses/6825a67ceccd.json

@@ -0,0 +1,50 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "http://localhost:8000/v1/v1/completions",
+    "headers": {},
+    "body": {
+      "model": "Qwen/Qwen3-0.6B",
+      "prompt": "I am feeling really sad today.",
+      "stream": false,
+      "extra_body": {
+        "guided_choice": [
+          "joy",
+          "sadness"
+        ]
+      }
+    },
+    "endpoint": "/v1/completions",
+    "model": "Qwen/Qwen3-0.6B"
+  },
+  "response": {
+    "body": {
+      "__type__": "openai.types.completion.Completion",
+      "__data__": {
+        "id": "cmpl-b4feb568934e44e7a8b7ac8071dce285",
+        "choices": [
+          {
+            "finish_reason": "stop",
+            "index": 0,
+            "logprobs": null,
+            "text": "sadness",
+            "stop_reason": null,
+            "prompt_logprobs": null
+          }
+        ],
+        "created": 1757524114,
+        "model": "Qwen/Qwen3-0.6B",
+        "object": "text_completion",
+        "system_fingerprint": null,
+        "usage": {
+          "completion_tokens": 3,
+          "prompt_tokens": 7,
+          "total_tokens": 10,
+          "completion_tokens_details": null,
+          "prompt_tokens_details": null
+        }
+      }
+    },
+    "is_streaming": false
+  }
+}