Log failed chat turns as chat entries

A turn whose agent raised wrote no user_logs row, so it only surfaced as
the agent's system error row. Every finished turn now writes its chat row,
at level error with the error when it failed, and linked to its trace; the
system row for the same traced activity is no longer listed twice.

tests/api/user/test_traces_routes.py

@@ -212,3 +212,39 @@ class TestSummaryFailure:
         assert response.status_code == 200
         (row,) = response.json["logs"]
         assert "trace" not in row
+
+
+class TestFailedChatTurns:
+    def _failed_turn(self, pg_conn, *, with_trace):
+        from docsgpt.storage.db.repositories.stack_logs import StackLogsRepository
+        from docsgpt.storage.db.repositories.user_logs import UserLogsRepository
+
+        StackLogsRepository(pg_conn).insert(
+            activity_id="act-chat", endpoint="stream", level="error", user_id="owner", query="q"
+        )
+        UserLogsRepository(pg_conn).insert(
+            user_id="owner",
+            endpoint="stream_answer",
+            data={
+                "question": "q",
+                "level": "error",
+                "error": "RuntimeError: upstream down",
+                "request_id": "req-f",
+            },
+        )
+        if with_trace:
+            _trace(pg_conn, request_id="req-f", activity_id="act-chat", status="error")
+
+    def test_failed_turn_is_one_chat_row_not_also_a_system_row(self, app, pg_conn):
+        self._failed_turn(pg_conn, with_trace=True)
+        rows = _logs(app, pg_conn, "owner", {}).json["logs"]
+        assert [r["event_type"] for r in rows] == ["chat"]
+        assert rows[0]["level"] == "error"
+        assert rows[0]["error"] == "RuntimeError: upstream down"
+        assert rows[0]["trace"]["status"] == "error"
+
+    def test_untraced_failures_keep_their_system_row(self, app, pg_conn):
+        """Rows from before traces (or with tracing off) have nothing to dedupe against."""
+        self._failed_turn(pg_conn, with_trace=False)
+        rows = _logs(app, pg_conn, "owner", {}).json["logs"]
+        assert sorted(r["event_type"] for r in rows) == ["chat", "system"]