fix(server): gate xorq infinite_request error_info on BUCKAROO_DEBUG

Closes #798. ``handle_infinite_request_xorq`` was returning
``traceback.format_exc()`` in the response's ``error_info`` field
unconditionally, leaking server-side source paths and stack frames to
WS clients in any production deployment.

This patch mirrors the pandas WS-handler pattern at
``websocket_handler.py:241-245``: log the full traceback server-side
(at ERROR level), but ship a generic ``"Request failed"`` to the client
unless ``BUCKAROO_DEBUG=1`` is set.

Pre-fix, the operator running a non-debug server *also* had no record
of the error (the xorq handler didn't log). This patch fixes that gap
too — same ``log.error`` shape as the pandas path.

Test from the previous commit now passes; full ``test_load_expr.py``
suite green.

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

Author: paddymul

buckaroo/server/xorq_loading.py

@@ -8,13 +8,21 @@
 """
 from __future__ import annotations
 
+import logging
+import os
 import traceback
 
 from buckaroo.xorq_buckaroo import (
     NoCleaningConfXorq, XorqAutocleaning, XorqDataflow, XorqDfStatsV2,
     XorqInfiniteSampling, _XORQ_ANALYSIS_KLASSES, _expr_count,
     window_to_parquet)
 
+# Mirrors ``websocket_handler._BUCKAROO_DEBUG`` — when set, error_info
+# carries the full traceback for local debugging. Without it, clients see
+# a generic message so source paths and stack frames don't leak.
+_BUCKAROO_DEBUG = os.environ.get("BUCKAROO_DEBUG", "").lower() in ("1", "true")
+log = logging.getLogger("buckaroo.server.xorq_loading")
+
 
 class XorqServerDataflow(XorqDataflow):
     """Headless XorqDataflow with infinite sampling.
@@ -87,5 +95,11 @@ def handle_infinite_request_xorq(xorq_dataflow: XorqServerDataflow,
         return ({"type": "infinite_resp", "key": payload_args, "data": [],
             "length": total_length}, parquet_bytes)
     except Exception:
+        tb = traceback.format_exc()
+        log.error("xorq infinite_request error: %s", tb)
+        # Mirrors the pandas-path gate in websocket_handler.py — clients
+        # in production runs see a generic message; only ``BUCKAROO_DEBUG``
+        # opens the source-leak channel.
         return ({"type": "infinite_resp", "key": payload_args, "data": [],
-            "length": 0, "error_info": traceback.format_exc()}, b"")
+            "length": 0,
+            "error_info": tb if _BUCKAROO_DEBUG else "Request failed"}, b"")