SwiftOpenAI v4.0.6

Adding convenient property in ResponseModel addressing #129

   /// Convenience property that aggregates all text output from output_text items in the output array.
   /// Similar to the outputText property in Python and JavaScript SDKs.
   public var outputText: String? {
      let outputTextItems = output.compactMap { outputItem -> String? in
         switch outputItem {
         case .message(let message):
            return message.content.compactMap { contentItem -> String? in
               switch contentItem {
               case .outputText(let outputText):
                  return outputText.text
               }
            }.joined()
         default:
            return nil
         }
      }
      
      return outputTextItems.isEmpty ? nil : outputTextItems.joined()
   }