chore: OpenAIMixin implements ModelsProtocolPrivate (#3662)

# What does this PR do?

add ModelsProtocolPrivate methods to OpenAIMixin

this will allow providers using OpenAIMixin to use a common interface


## Test Plan

ci w/ new tests

tests/integration/recordings/responses/53d2488c9ea9.json

@@ -0,0 +1,40 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "http://localhost:11434/api/generate",
+    "headers": {},
+    "body": {
+      "model": "llama3.2:3b-instruct-fp16",
+      "options": {
+        "temperature": 0.0001,
+        "top_p": 0.9
+      },
+      "stream": true
+    },
+    "endpoint": "/api/generate",
+    "model": "llama3.2:3b-instruct-fp16"
+  },
+  "response": {
+    "body": [
+      {
+        "__type__": "ollama._types.GenerateResponse",
+        "__data__": {
+          "model": "llama3.2:3b-instruct-fp16",
+          "created_at": "2025-10-01T20:38:48.732564955Z",
+          "done": true,
+          "done_reason": "load",
+          "total_duration": null,
+          "load_duration": null,
+          "prompt_eval_count": null,
+          "prompt_eval_duration": null,
+          "eval_count": null,
+          "eval_duration": null,
+          "response": "",
+          "thinking": null,
+          "context": null
+        }
+      }
+    ],
+    "is_streaming": true
+  }
+}