Fix handling of OpenAI messages with content and multi-content.

Previously, OpenAI messages containing both `Content` and `MultiContent` could cause inconsistent behavior. This update ensures `Content` is converted into a `MultiContent` entry to maintain compatibility.

llm.go

@@ -106,6 +106,15 @@ func (m Message) toChatCompletionMessages() []openai.ChatCompletionMessage {
 		}
 	}
 
+	// openai does not support messages with both content and multi-content
+	if len(res.MultiContent) > 0 && res.Content != "" {
+		res.MultiContent = append([]openai.ChatMessagePart{{
+			Type: "text",
+			Text: res.Content,
+		}}, res.MultiContent...)
+		res.Content = ""
+	}
+
 	return []openai.ChatCompletionMessage{res}
 }