Route prepared export policy through platform services

src/platform_windows/windows_platform_services.cpp

@@ -448,6 +448,11 @@ public:
         invoke_selected_path(path, callback);
     }
 
+    [[nodiscard]] bool uses_prepared_file_writes() override
+    {
+        return false;
+    }
+
     [[nodiscard]] pp::platform::PreparedFileTarget prepare_writable_file(
         std::string_view type,
         std::string_view default_name,