tests: add MockDDS and use it in GTest smoke-tests to simulate publish/subscribe flows

Lts/DDS/DdsUnitTestsPlaceholder/mocks/MockDDS.h

@@ -0,0 +1,11 @@
+#pragma once
+#include <string>
+
+namespace MockDDS {
+
+// Very small mock: single-writer, single-last-message per topic store.
+void publish(const std::string &topic, const std::string &msg);
+std::string take_last(const std::string &topic);
+void clear_all();
+
+}