Attachment fetchData lost after debounce/queue serialization

[steven-moneyforward]

Bug Description

When using the debounce or queue concurrency strategy, Attachment.fetchData closures are lost during message serialization. enqueue
calls JSON.stringify(entry) which strips functions. After rehydrateMessage, all attachments have fetchData: undefined — handlers cannot
download files/images.

Steps to Reproduce

1. Create a Chat instance with concurrency: { strategy: 'debounce', debounceMs: 2000 }
2. Register an onSubscribedMessage handler that accesses message.attachments[0].fetchData
3. Send a message with an image attachment from Slack
4. Message enters handleQueueOrDebounce → enqueue → JSON.stringify strips fetchData
5. After debounce delay, dequeue → rehydrateMessage → attachments[].fetchData is undefined
6. Handler throws: TypeError: fetchData is not a function

Expected Behavior

After dequeue + rehydrate, attachment.fetchData should be a working function. The attachment's url, name, mimeType survive serialization — only
fetchData (a closure) is lost. rehydrateMessage should recreate fetchData from the adapter using att.url.

Actual Behavior

attachment.fetchData is undefined after rehydration. All file/image downloads fail for debounced messages. Works correctly with 'drop' and
'concurrent' strategies (no serialization).

Code Sample

import { Chat } from 'chat';
  import { SlackAdapter } from '@chat-adapter/slack';

  const chat = new Chat({
    adapters: [new SlackAdapter({ botToken: '...' })],
    concurrency: { strategy: 'debounce', debounceMs: 2000 },
  });

  chat.onSubscribedMessage(async (thread, message) => {
    const att = message.attachments.find(a => a.type === 'image');
    if (att) {
      console.log(att.url);       // ✅ "https://files.slack.com/..." (survives serialization)
      console.log(att.fetchData); // ❌ undefined (closure lost after JSON.stringify)
      await att.fetchData();      // 💥 TypeError: fetchData is not a function
    }
  });

Chat SDK Version

latest

Node.js Version

4.23.0

Platform Adapter

No response

Operating System

None

Additional Context

No response