Allow extra fields in ChatCompletionRequest to fix 422 errors

- Add model_config with extra='allow' to ChatCompletionRequest
- Fixes 422 validation errors when OpenAI-compatible clients send unknown fields
- Maintains backward compatibility with all existing fields
- Tested with unknown_field and extra_param - request succeeds

app/models/openai_models.py

@@ -32,6 +32,8 @@ class ToolCall(BaseModel):
 
 class ChatCompletionRequest(BaseModel):
     """OpenAI chat completion request."""
+    model_config = {"extra": "allow"}
+    
     model: str
     messages: List[ChatMessage]
     temperature: Optional[float] = Field(default=1.0, ge=0, le=2)