Refactor error handling in message sending loop

Changed from range over messages to range with index to correctly update sendError field in the original messages slice. This prevents shadowing issues and ensures proper error logging for each message.

client_120.go

@@ -24,9 +24,9 @@ func (c *Client) Send(messages ...*Msg) (returnErr error) {
 		returnErr = errors.Join(errs...)
 	}()
 
-	for _, message := range messages {
+	for id, message := range messages {
 		if sendErr := c.sendSingleMsg(message); sendErr != nil {
-			message.sendError = sendErr
+			messages[id].sendError = sendErr
 			errs = append(errs, sendErr)
 		}
 	}