test(server): failing test for xorq infinite_request traceback leak

#798: ``handle_infinite_request_xorq`` returns ``traceback.format_exc()``
in the ``error_info`` field unconditionally — leaking server source
paths and stack frames to WS clients. The pandas-path handler in
``websocket_handler.py`` correctly gates this on ``BUCKAROO_DEBUG``.

Test triggers the exception path with a sort key that doesn't exist in
``merged_sd`` (raises ``KeyError``), then asserts ``error_info`` does
not start with ``"Traceback"``. Fails today; will pass once the xorq
handler gates the traceback behind ``BUCKAROO_DEBUG``.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: paddymul

tests/unit/server/test_load_expr.py

@@ -141,6 +141,46 @@ async def test_ws_search_pushdown(self):
         finally:
             shutil.rmtree(builds_root, ignore_errors=True)
 
+    @tornado.testing.gen_test
+    async def test_xorq_infinite_request_error_info_no_traceback(self):
+        """Regression for #798: the xorq path put ``traceback.format_exc()``
+        into ``error_info`` unconditionally — leaking server-side source
+        paths to WS clients. Pandas path gates this behind
+        ``BUCKAROO_DEBUG``; xorq must too.
+
+        Trigger the exception with a sort column that doesn't exist in
+        ``merged_sd`` — raises ``KeyError`` inside
+        ``handle_infinite_request_xorq``.
+        """
+        builds_root = tempfile.mkdtemp()
+        try:
+            build_path = _build_expr_dir(builds_root)
+            await _post(self.get_http_port(), "/load_expr",
+                {"session": "lx-leak", "build_dir": build_path})
+
+            ws = await tornado.websocket.websocket_connect(
+                f"ws://localhost:{self.get_http_port()}/ws/lx-leak")
+            await ws.read_message()  # discard initial_state
+
+            ws.write_message(json.dumps({
+                "type": "infinite_request",
+                "payload_args": {"start": 0, "end": 5,
+                    "sort": "nonexistent_col", "sort_direction": "asc",
+                    "sourceName": "default", "origEnd": 5}}))
+
+            r = json.loads(await ws.read_message())
+            self.assertEqual(r["type"], "infinite_resp")
+            self.assertIn("error_info", r)
+            # The bug: pre-fix this carries a full traceback starting
+            # with "Traceback (most recent call last):\n  File ...".
+            # Production runs shouldn't leak source paths to clients.
+            self.assertFalse(r["error_info"].startswith("Traceback"),
+                f"error_info leaked traceback to client (first 200 chars): "
+                f"{r['error_info'][:200]!r}")
+            ws.close()
+        finally:
+            shutil.rmtree(builds_root, ignore_errors=True)
+
     @tornado.testing.gen_test
     async def test_session_reuse_xorq_then_pandas(self):
         """A client that POSTs /load_expr and then POSTs /load with the