tests: richer MockDDS (subscriptions, queues, seq) and link into Qt/GTest smoke-tests

Lts/DDS/DdsUnitTestsPlaceholder/mocks/MockDDS.h

@@ -1,11 +1,26 @@
 #pragma once
 #include <string>
+#include <functional>
+#include <vector>
 
 namespace MockDDS {
 
-// Very small mock: single-writer, single-last-message per topic store.
+// Richer mock for migration smoke-tests:
+// - publish(topic, payload)
+// - subscribe(topic, callback) -> subscription id
+// - unsubscribe(topic, subscription id)
+// - take_last(topic), get_queue(topic)
+// - clear_all()
+// - sequence numbers per topic (as simple integer appended to payload meta)
+
+using Subscriber = std::function<void(const std::string &payload)>;
+using SubscriptionId = unsigned long;
+
 void publish(const std::string &topic, const std::string &msg);
+SubscriptionId subscribe(const std::string &topic, Subscriber cb);
+void unsubscribe(const std::string &topic, SubscriptionId id);
 std::string take_last(const std::string &topic);
+std::vector<std::string> get_queue(const std::string &topic);
 void clear_all();
 
 }