Core/Creature: implement initial SummonInfo API for handling summons in the future

src/server/game/Entities/Unit/Unit.cpp

@@ -79,6 +79,7 @@
 #include "SpellInfo.h"
 #include "SpellMgr.h"
 #include "SpellPackets.h"
+#include "SummonInfo.h"
 #include "TemporarySummon.h"
 #include "Totem.h"
 #include "Transport.h"
@@ -8818,6 +8819,23 @@ uint32 Unit::GetCreatureTypeMask() const
     return (creatureType >= 1) ? (1 << (creatureType - 1)) : 0;
 }
 
+void Unit::RegisterSummon(SummonInfo* summon)
+{
+    if (!summon)
+        return;
+
+    if (!advstd::ranges::contains(_activeSummons, summon))
+        _activeSummons.push_back(summon);
+}
+
+void Unit::UnregisterSummon(SummonInfo* summon)
+{
+    if (!summon)
+        return;
+
+    std::erase(_activeSummons, summon);
+}
+
 void Unit::SetShapeshiftForm(ShapeshiftForm form)
 {
     SetByteValue(UNIT_FIELD_BYTES_2, UNIT_BYTES_2_OFFSET_SHAPESHIFT_FORM, form);
@@ -9607,6 +9625,9 @@ void Unit::RemoveFromWorld()
             }
         }
 
+        // Clear all active summon references. If we are about to switch map instances, we want to make sure that we leave all summons behind so we won't do threadunsafe operations
+        _activeSummons.clear();
+
         WorldObject::RemoveFromWorld();
         m_duringRemoveFromWorld = false;
     }