Qt6 Migration: BUILD_RTI guards und CMake fixes

- BUILD_RTI conditional compilation für RTI Connext DDS dependencies
- Namespace fixes in core_dtc.h (Core:: → DDS::Core::)
- CMakeLists.txt: BUILD_RTI option und conditional dependencies
- Handler minelayer: namespace closing mit BUILD_RTI guard
- Build erfolgreich mit BUILD_RTI=OFF validiert

Signed-off-by: Torsten Wierschin <torsten.wierschin@life-maps.com>

qt6-after/qt6-migrated-legatus/tests/test_migration_stubs.cpp

@@ -8,6 +8,7 @@
  */
 
 #include <gtest/gtest.h>
+#include <QString>
 
 // Test that the test framework is working
 TEST(MigrationStubs, TestFrameworkWorks)
@@ -103,7 +104,7 @@ TEST(MigrationStubs, ComponentMonitorDtcCompiles)
 
 TEST(MigrationStubs, CoreDtcCompiles)
 {
-    using namespace Core::Interface;
+    using namespace DDS::Core::Interface;
     EXPECT_TRUE(true);
 }