refactor(Core/Combat): Port TrinityCore heap-based threat system (#24715)

Co-authored-by: blinkysc <blinkysc@users.noreply.github.com>
Co-authored-by: Treeston <treeston.mmoc@gmail.com>
Co-authored-by: killerwife <killerwife@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/server/game/Entities/Pet/Pet.cpp

@@ -2318,6 +2318,7 @@ bool Pet::Create(ObjectGuid::LowType guidlow, Map* map, uint32 phaseMask, uint32
     // Force regen flag for player pets, just like we do for players themselves
     SetUnitFlag2(UNIT_FLAG2_REGENERATE_POWER);
     SetSheath(SHEATH_STATE_MELEE);
+    GetThreatMgr().Initialize();
 
     return true;
 }