fix(OllamaAPI): Add validation to ensure last message is from user role

• Validate last message role is "user"
• Raise 400 error for invalid role
• Improve API request validation
• Prevent invalid message sequences

lightrag/api/routers/ollama_api.py

@@ -483,6 +483,12 @@ class OllamaAPI:
                 if not messages:
                     raise HTTPException(status_code=400, detail="No messages provided")
 
+                # Validate that the last message is from a user
+                if messages[-1].role != "user":
+                    raise HTTPException(
+                        status_code=400, detail="Last message must be from user role"
+                    )
+
                 # Get the last message as query and previous messages as history
                 query = messages[-1].content
                 # Convert OllamaMessage objects to dictionaries