Add support for 'developer' role in chat messages

- Add 'developer' to accepted role literals in ChatMessage model
- Existing normalization in chat.py converts developer → system before watsonx API call
- Fixes 422 validation errors when clients send developer role messages
- Maintains backward compatibility with all existing roles

app/models/openai_models.py

@@ -10,7 +10,7 @@ from pydantic import BaseModel, Field
 
 class ChatMessage(BaseModel):
     """A chat message in the conversation."""
-    role: Literal["system", "user", "assistant", "function", "tool"]
+    role: Literal["system", "user", "assistant", "function", "tool", "developer"]
     content: Optional[str] = None
     name: Optional[str] = None
     function_call: Optional[Dict[str, Any]] = None