fix(slack): read thread_id from metadata for in-thread routing

[boyleryan]

Summary

Slack thread replies — progress messages, typing indicators, session hygiene
warnings, and other auxiliary sends from _process_message_background() — land
in the top-level DM window instead of the conversation thread.

Root Cause

The base adapter and gateway construct thread metadata with a thread_id key:

• gateway/platforms/base.py:691: _thread_metadata = {"thread_id": event.source.thread_id}
• gateway/run.py: _progress_metadata = {"thread_id": source.thread_id}

But SlackAdapter.send() only checks for thread_ts:

• gateway/platforms/slack.py:163: metadata.get("thread_ts")

The value is correct — it's the original Slack thread_ts, stored as thread_id
in MessageSource at slack.py:530. The key name just doesn't match what
send() looks for.

The Telegram adapter already handles this correctly — telegram.py:218 reads
metadata.get("thread_id").

Changes

• gateway/platforms/slack.py: Accept both thread_ts and thread_id from
  metadata in send(), preferring thread_ts when both are present. This aligns
  with the Telegram adapter's approach of reading the canonical thread_id key.
• tests/gateway/test_slack.py: Add TestSendThreading class with 4 tests:
  thread_id-only metadata, thread_ts-only metadata, both-keys precedence,
  and reply_to precedence over metadata.

How to Test

1. Connect a Slack workspace
2. Start a threaded conversation with the bot
3. Send a message that triggers tool use (e.g., web search)
4. Verify progress messages appear in the thread, not the main DM window

Automated: pytest tests/gateway/test_slack.py::TestSendThreading -v

Platforms Tested

• Slack (primary fix, live-verified)

Related

• Supersedes the Slack threading portion of fix(gateway): support threaded metadata in typing loop #857 and fix: accept metadata in gateway helper signatures #851 (both closed without merge)

Known Related Issue (not addressed here)

The Slack media methods (send_image, send_document, send_video,
send_voice, send_image_file) accept **kwargs from the base adapter but
don't extract metadata for threading. Media sent during threaded conversations
may land outside the thread. This is a separate issue affecting media method
implementations across adapters and should be addressed in a follow-up.

[teknium1]

Superseded by PR #1103 (merged), which addressed the same thread_id/thread_ts key mismatch fix plus additional threading improvements.