Summary

Fixes #268

When handling DM (direct message) events, the Slack adapter set threadTs to an empty string when event.thread_ts was undefined:

// Before
const threadTs = isDM ? event.thread_ts || "" : event.thread_ts || event.ts;

This caused invalid_thread_ts errors from the Slack API when attempting to stream responses back to DMs via chatStream.

The fix uses event.ts as the fallback for all message types:

// After
const threadTs = event.thread_ts || event.ts;

This ensures the bot replies in a thread under the original DM message, matching the behavior for channel messages.

Behavioral Note

This changes how top-level DM messages interact with openDM() subscriptions:

• Before: top-level DM threadId was slack:CHANNEL: (empty threadTs), which exactly matched openDM() subscription IDs → messages routed to onSubscribedMessage()
• After: top-level DM threadId is slack:CHANNEL:event.ts → messages now route to onNewMention() instead of onSubscribedMessage()

Apps using openDM() and relying on exact subscription matching should be aware of this change. Messages are still delivered correctly — only the routing handler changes.

Test plan

• Updated unit tests and integration tests to verify the new behavior
• Verified the fix resolves the invalid_thread_ts error in a production deployment
• Bot now correctly replies to DMs with streaming responses in a thread
• pnpm validate passes cleanly