refactor: enhance Dify integration with improved validation and message processing

This commit refines the Dify integration by updating the controller and service logic for better functionality and maintainability. Key changes include:
- Added Dify-specific validation in the createBot method to prevent duplicate entries.
- Simplified comments for clarity and removed unused methods in DifyController.
- Enhanced message processing in DifyService to handle audio messages more effectively and improve error handling.
- Updated DifyDto and DifySettingDto to streamline properties and improve clarity.

These updates contribute to a more robust and maintainable Dify integration.

src/api/integrations/chatbot/dify/dto/dify.dto.ts

@@ -3,12 +3,11 @@ import { $Enums } from '@prisma/client';
 import { BaseChatbotDto, BaseChatbotSettingDto } from '../../base-chatbot.dto';
 
 export class DifyDto extends BaseChatbotDto {
-  // Dify specific fields
   botType?: $Enums.DifyBotType;
   apiUrl?: string;
   apiKey?: string;
 }
 
 export class DifySettingDto extends BaseChatbotSettingDto {
-  // Dify specific fields
+  difyIdFallback?: string;
 }