fix(slack): persist trace_message_ts so web-handoff updates the trace reply

Re-land upstream #1630 onto the modular structure. The first-mention
store_slack_run_mapping call did not pass trace_message_ts, so it was
never persisted (nothing to preserve from on first mention) and
_notify_slack_web_handoff always skipped the trace-reply update on web
handoff. Pass it through and cover it with a test.

Refs #80, upstream #1630

agent/webhooks/slack.py

@@ -258,6 +258,7 @@ async def process_slack_mention(event_data: dict[str, Any], repo_config: dict[st
                 run_id,
                 message_ts=trace_message_ts,
                 triggering_user_id=user_id,
+                trace_message_ts=trace_message_ts,
             )
     else:
         webapp.logger.info(

tests/test_slack_context.py

@@ -638,6 +638,44 @@ def test_process_slack_mention_uses_vision_fallback_for_image_thread(
     assert "does not support image input" not in content[0]["text"]
 
 
+def test_process_slack_mention_persists_trace_message_ts_on_first_mention(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    captured: dict[str, object] = {}
+    _setup_slack_mention_fakes(monkeypatch, captured)
+
+    async def fake_thread_exists(thread_id: str) -> bool:
+        return False
+
+    async def fake_post_slack_trace_reply(channel_id: str, thread_ts: str, thread_id: str) -> str:
+        return "1700000000.999999"
+
+    async def fake_store_slack_run_mapping(*args, **kwargs) -> None:
+        captured["run_mapping"] = {"args": args, "kwargs": kwargs}
+
+    monkeypatch.setattr(webapp, "_thread_exists", fake_thread_exists)
+    monkeypatch.setattr(webapp, "post_slack_trace_reply", fake_post_slack_trace_reply)
+    monkeypatch.setattr(webapp, "store_slack_run_mapping", fake_store_slack_run_mapping)
+
+    asyncio.run(
+        webapp.process_slack_mention(
+            {
+                "channel_id": "C123",
+                "thread_ts": "1700000000.000100",
+                "event_ts": "1700000000.000200",
+                "user_id": "U123",
+                "text": "<@UBOT> continue on the branch",
+                "bot_user_id": "UBOT",
+            },
+            {"owner": "langchain-ai", "name": "open-swe"},
+        )
+    )
+
+    run_mapping = captured["run_mapping"]
+    assert isinstance(run_mapping, dict)
+    assert run_mapping["kwargs"]["trace_message_ts"] == "1700000000.999999"
+
+
 def test_process_slack_mention_skips_trace_reply_on_followup_mention(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None: