fix: use StopMCRun instead of StopRun in GeneratePrimaries

StopRun() calls exit(0) which bypasses destructors and prevents
proper file closure. StopMCRun() signals the MC engine to stop
gracefully after the current event.

Author: olantwin

fairroot/base/sim/FairMCApplication.cxx

@@ -977,7 +977,7 @@ void FairMCApplication::GeneratePrimaries()
     if (fEvGen) {
         //    LOG(debug) << "FairMCApplication::GeneratePrimaries()";
         if (!fEvGen->GenerateEvent(fStack.get())) {
-            StopRun();
+            StopMCRun();
         }
     }
 }