feat: implement Anthropic native PDF support with document preservation

- Add comprehensive debug logging throughout PDF processing pipeline
- Refactor attachment processing to separate image and document handling
- Create distinct addImageURLs(), addDocuments(), and processAttachments() methods
- Fix critical bugs in stream handling and parameter passing
- Add streamToBuffer utility for proper stream-to-buffer conversion
- Remove api/agents submodule from repository

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

api/app/clients/BaseClient.js

@@ -1233,7 +1233,7 @@ class BaseClient {
         {},
       );
 
-      await this.addImageURLs(message, files, this.visionMode);
+      await this.processAttachments(message, files, this.visionMode);
 
       this.message_file_map[message.messageId] = files;
       return message;