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/6cf57d8dfcfc.json

@@ -0,0 +1,86 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "http://localhost:8000/v1/v1/completions",
+    "headers": {},
+    "body": {
+      "model": "Qwen/Qwen3-0.6B",
+      "prompt": "<|begin_of_text|>Complete the sentence: Micheael Jordan is born in ",
+      "logprobs": 1,
+      "stream": false,
+      "temperature": 0.0,
+      "max_tokens": 5
+    },
+    "endpoint": "/v1/completions",
+    "model": "Qwen/Qwen3-0.6B"
+  },
+  "response": {
+    "body": {
+      "__type__": "openai.types.completion.Completion",
+      "__data__": {
+        "id": "cmpl-b114bec84b5a430ba94520ac9e7a6d8d",
+        "choices": [
+          {
+            "finish_reason": "length",
+            "index": 0,
+            "logprobs": {
+              "text_offset": [
+                0,
+                1,
+                2,
+                3,
+                4
+              ],
+              "token_logprobs": [
+                -0.03187580779194832,
+                -0.014971747994422913,
+                -0.3393988609313965,
+                -1.2200194597244263,
+                -0.762129545211792
+              ],
+              "tokens": [
+                "1",
+                "9",
+                "5",
+                "8",
+                ","
+              ],
+              "top_logprobs": [
+                {
+                  "1": -0.03187580779194832
+                },
+                {
+                  "9": -0.014971747994422913
+                },
+                {
+                  "5": -0.3393988609313965
+                },
+                {
+                  "8": -1.2200194597244263
+                },
+                {
+                  ",": -0.762129545211792
+                }
+              ]
+            },
+            "text": "1958,",
+            "stop_reason": null,
+            "prompt_logprobs": null
+          }
+        ],
+        "created": 1757524133,
+        "model": "Qwen/Qwen3-0.6B",
+        "object": "text_completion",
+        "system_fingerprint": null,
+        "usage": {
+          "completion_tokens": 5,
+          "prompt_tokens": 18,
+          "total_tokens": 23,
+          "completion_tokens_details": null,
+          "prompt_tokens_details": null
+        }
+      }
+    },
+    "is_streaming": false
+  }
+}