🚀 fix(model-sync): avoid unnecessary upstream fetch while keeping overwrite updates working

- Only short-circuit when there are no missing models AND no overwrite fields requested
- Preserve overwrite behavior even when the missing-model list is empty
- Always return empty arrays (not null) for list fields to keep API responses stable
- Clarify SyncUpstreamModels behavior in comments (create missing models + optional overwrite updates)

service/convert.go

@@ -401,7 +401,7 @@ func StreamResponseOpenAI2Claude(openAIResponse *dto.ChatCompletionsStreamRespon
 						},
 					})
 				}
-				
+
 				if len(toolCall.Function.Arguments) > 0 {
 					claudeResponses = append(claudeResponses, &dto.ClaudeResponse{
 						Index: &idx,