Keep tool output out of span errors and fix trace status and listing

Tool results, denial comments and tool exception text now reach a span only
as a capture-gated preview; span.error is a fixed message, since it is
stored and exported regardless of content settings. A turn whose stream
yields an error event is recorded as failed. Search traces are listed by a
query copied into the small summary column, so the Logs timeline never
reads the spans JSONB. Adds GraphRAG span tests.

tests/api/answer/test_trace_wiring.py

@@ -132,6 +132,17 @@ class TestTraceLifecycle:
         assert any('"type": "error"' in s for s in stream)
         assert flushed[0].status == "error"
 
+    def test_yielded_error_marks_trace_error(self, flask_app, mock_mongo_db):
+        """A failed workflow node yields an error event instead of raising."""
+        from docsgpt.api.answer.routes.base import BaseAnswerResource
+
+        with flask_app.app_context(), _captured_flushes() as flushed:
+            _run(
+                BaseAnswerResource(),
+                _agent([{"type": "error", "error": "node failed"}]),
+            )
+        assert flushed[0].status == "error"
+
     def test_abandoned_stream_still_flushes(self, flask_app, mock_mongo_db):
         from docsgpt.api.answer.routes.base import BaseAnswerResource