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/0547d0909f24.json

@@ -0,0 +1,53 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "https://api.cerebras.ai/v1/v1/completions",
+    "headers": {},
+    "body": {
+      "model": "llama-3.3-70b",
+      "prompt": "Respond to this question and explain your answer. Complete the sentence using one word: Roses are red, violets are ",
+      "stream": false,
+      "extra_body": {}
+    },
+    "endpoint": "/v1/completions",
+    "model": "llama-3.3-70b"
+  },
+  "response": {
+    "body": {
+      "__type__": "openai.types.completion.Completion",
+      "__data__": {
+        "id": "chatcmpl-6438a448-bbbd-4da1-af88-19390676b0e9",
+        "choices": [
+          {
+            "finish_reason": "stop",
+            "index": 0,
+            "logprobs": null,
+            "text": " blue, sugar is white, but my heart is ________________________.\nA) black\nB) pink\nC) blank\nD) broken\nMy answer is D) broken. This is because the traditional romantic poem has a positive tone until it comes to the heart, which represents the speaker's emotional state. The word \"broken\" shows that the speaker is hurting, which adds a element of sadness to the poem. This is a typical way to express sorrow or longing in poetry.\nThe best answer is D.<|eot_id|>"
+          }
+        ],
+        "created": 1758191351,
+        "model": "llama-3.3-70b",
+        "object": "text_completion",
+        "system_fingerprint": "fp_c5ec625e72d41732d8fd",
+        "usage": {
+          "completion_tokens": 105,
+          "prompt_tokens": 26,
+          "total_tokens": 131,
+          "completion_tokens_details": null,
+          "prompt_tokens_details": {
+            "audio_tokens": null,
+            "cached_tokens": 0
+          }
+        },
+        "time_info": {
+          "queue_time": 0.00016155,
+          "prompt_time": 0.001595551,
+          "completion_time": 0.107480394,
+          "total_time": 0.11038637161254883,
+          "created": 1758191351
+        }
+      }
+    },
+    "is_streaming": false
+  }
+}