fix: update response body handling in OpenAI relay format

relay/channel/openai/relay-openai.go

@@ -216,7 +216,16 @@ func OpenaiHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http.Respo
 
 	switch info.RelayFormat {
 	case types.RelayFormatOpenAI:
-		if forceFormat || usageModified {
+		if usageModified {
+			var bodyMap map[string]interface{}
+			err = common.Unmarshal(responseBody, &bodyMap)
+			if err != nil {
+				return nil, types.NewOpenAIError(err, types.ErrorCodeBadResponseBody, http.StatusInternalServerError)
+			}
+			bodyMap["usage"] = simpleResponse.Usage
+			responseBody, _ = common.Marshal(bodyMap)
+		}
+		if forceFormat {
 			responseBody, err = common.Marshal(simpleResponse)
 			if err != nil {
 				return nil, types.NewError(err, types.ErrorCodeBadResponseBody)