fix(issue-sink): stringify triggering_event_id before JSON encode

IssueCoreRestSink.emit() passed task_spec.triggering_event_id straight
into the httpx json= payload. When the field is a UUID object (rather
than a string), httpx's JSON encoder raised
"TypeError: Object of type UUID is not JSON serializable", failing the
emission. Guard with str(), preserving None for optional event ids.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/activity_core/issue_sink.py

@@ -66,7 +66,11 @@ class IssueCoreRestSink(IssueSink):
             "due_in_days": task_spec.due_in_days,
             "source_type": task_spec.source_type,
             "source_id": task_spec.source_id,
-            "triggering_event_id": task_spec.triggering_event_id,
+            "triggering_event_id": (
+                str(task_spec.triggering_event_id)
+                if task_spec.triggering_event_id is not None
+                else None
+            ),
             "activity_definition_id": task_spec.activity_definition_id,
         }
         resp = httpx.post(