fix(slack): preserve string thread context in queue + DM route (#23804)

* fix(slack): preserve thread_ts in queue drain and deliveryContext

Two related fixes for Slack thread reply routing:

1. Queue drain drops string thread_ts (#11195)
   - `typeof threadId === "number"` in drain.ts only matches Telegram numeric
     topic IDs. Slack thread_ts is a string like "1770474140.187459" which
     fails the check, causing threadKey to become empty.
   - Changed to `threadId != null && threadId !== ""` to accept both number
     and string thread IDs.
   - Applies to all 3 occurrences in drain.ts: cross-channel detection,
     thread key building, and collected originatingThreadId extraction.

2. DM deliveryContext missing thread_ts (#10837)
   - updateLastRoute calls for Slack DMs in both prepare.ts and dispatch.ts
     built deliveryContext without threadId, so the session's delivery context
     never included thread_ts for DM threads.
   - Added threadId from threadContext.messageThreadId / ctxPayload.MessageThreadId
     to both updateLastRoute call sites.

Tests: 3 new cases in queue.collect-routing.test.ts
- Collects messages with matching string thread_ts (same Slack thread)
- Separates messages with different string thread_ts (different threads)
- Treats empty string threadId same as absent

Closes #10837, closes #11195

* fix(slack): preserve string thread context in queue + DM route updates

---------

Co-authored-by: RobClawd <clawd@RobClawds-Mac-mini.local>

src/slack/monitor/message-handler/dispatch.ts

@@ -80,6 +80,7 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
         channel: "slack",
         to: `user:${message.user}`,
         accountId: route.accountId,
+        threadId: prepared.ctxPayload.MessageThreadId,
       },
       ctx: prepared.ctxPayload,
     });