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

Lts/DDS/DdsUnitTestsPlaceholder/mocks/MockDDS.cpp

@@ -0,0 +1,27 @@
+#include "MockDDS.h"
+#include <map>
+#include <mutex>
+
+namespace MockDDS {
+
+static std::map<std::string, std::string> g_last_message;
+static std::mutex g_mtx;
+
+void publish(const std::string &topic, const std::string &msg) {
+    std::lock_guard<std::mutex> lk(g_mtx);
+    g_last_message[topic] = msg;
+}
+
+std::string take_last(const std::string &topic) {
+    std::lock_guard<std::mutex> lk(g_mtx);
+    auto it = g_last_message.find(topic);
+    if (it == g_last_message.end()) return std::string();
+    return it->second;
+}
+
+void clear_all() {
+    std::lock_guard<std::mutex> lk(g_mtx);
+    g_last_message.clear();
+}
+
+} // namespace MockDDS