fix(backend): consume webview tickets atomically and fail closed on Redis errors.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_mp_webview_ticket.py

@@ -1,4 +1,3 @@
-import json
 from unittest.mock import patch
 
 import pytest
@@ -22,6 +21,9 @@ class FakeRedis:
     def delete(self, key):
         return 1 if self.data.pop(key, None) is not None else 0
 
+    def getdel(self, key):
+        return self.data.pop(key, None)
+
 
 def test_issue_and_consume_once():
     fake = FakeRedis()
@@ -44,3 +46,9 @@ def test_issue_without_redis_raises():
     with patch.object(svc, "get_sync_redis_client", return_value=None):
         with pytest.raises(RuntimeError, match="Redis"):
             svc.issue_ticket("u1", "sid-1")
+
+
+def test_consume_without_redis_raises():
+    with patch.object(svc, "get_sync_redis_client", return_value=None):
+        with pytest.raises(RuntimeError, match="Redis"):
+            svc.consume_ticket("any-ticket")