feat: update Cerebras inference provider to support dynamic model listing

- update Cerebras to use OpenAIMixin
- enable openai completions tests
- enable openai chat completions tests
- disable with n > 1 tests
- add recording for --setup cerebras --subdirs inference --pattern openai

test with: `./scripts/integration-tests.sh --stack-config server:ci-tests --setup cerebras --subdirs inference --pattern openai`

tests/integration/recordings/responses/b459f403a5ae.json

@@ -0,0 +1,66 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "https://api.cerebras.ai/v1/v1/chat/completions",
+    "headers": {},
+    "body": {
+      "model": "llama-3.3-70b",
+      "messages": [
+        {
+          "role": "user",
+          "content": "Which planet has rings around it with a name starting with letter S?"
+        }
+      ],
+      "stream": false
+    },
+    "endpoint": "/v1/chat/completions",
+    "model": "llama-3.3-70b"
+  },
+  "response": {
+    "body": {
+      "__type__": "openai.types.chat.chat_completion.ChatCompletion",
+      "__data__": {
+        "id": "chatcmpl-e9e83004-bcd0-47f8-97c3-8e3d789a6573",
+        "choices": [
+          {
+            "finish_reason": "stop",
+            "index": 0,
+            "logprobs": null,
+            "message": {
+              "content": "The planet with rings around it that starts with the letter S is Saturn. Saturn's rings are one of the most prominent and well-known ring systems in our solar system.",
+              "refusal": null,
+              "role": "assistant",
+              "annotations": null,
+              "audio": null,
+              "function_call": null,
+              "tool_calls": null
+            }
+          }
+        ],
+        "created": 1758191362,
+        "model": "llama-3.3-70b",
+        "object": "chat.completion",
+        "service_tier": null,
+        "system_fingerprint": "fp_c5ec625e72d41732d8fd",
+        "usage": {
+          "completion_tokens": 35,
+          "prompt_tokens": 49,
+          "total_tokens": 84,
+          "completion_tokens_details": null,
+          "prompt_tokens_details": {
+            "audio_tokens": null,
+            "cached_tokens": 0
+          }
+        },
+        "time_info": {
+          "queue_time": 0.00091223,
+          "prompt_time": 0.00239449,
+          "completion_time": 0.013951346,
+          "total_time": 0.01872849464416504,
+          "created": 1758191362
+        }
+      }
+    },
+    "is_streaming": false
+  }
+}