fix: correct API response parsing in workflow integration tests

- API returns {"vendors": [...]} not a direct list
- Update test to access vendors_response["vendors"] correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/workflows/test_integration.py

@@ -47,8 +47,11 @@ class TestIntegrationFlows:
         # 3. List all vendors
         response = client.get("/api/v1/admin/vendors", headers=admin_headers)
         assert response.status_code == 200
-        vendors = response.json()
-        assert any(v["vendor_code"] == vendor["vendor_code"] for v in vendors)
+        vendors_response = response.json()
+        assert any(
+            v["vendor_code"] == vendor["vendor_code"]
+            for v in vendors_response["vendors"]
+        )
 
     def test_admin_product_listing_workflow(
         self, client, admin_headers, test_marketplace_product