test(yuanbao): add missing patch import to pipeline tests

The salvaged refactor's new tests use unittest.mock.patch (25 call sites)
but the import line only brought in AsyncMock and MagicMock, so 10 of the
new tests failed with NameError. Add patch to the import.

tests/test_yuanbao_pipeline.py

@@ -20,7 +20,7 @@ if _REPO_ROOT not in sys.path:
     sys.path.insert(0, _REPO_ROOT)
 
 import pytest
-from unittest.mock import AsyncMock, MagicMock
+from unittest.mock import AsyncMock, MagicMock, patch
 
 from gateway.platforms.yuanbao import (
     InboundContext,