fix(Core/Unit): fixup port of Combat/threat system rewrite (#27347)

Co-authored-by: Treeston <[email protected]>

src/server/game/Combat/ThreatManager.cpp

@@ -400,6 +400,15 @@ void ThreatManager::AddThreat(Unit* target, float amount, SpellInfo const* spell
             return;
     }
 
+    // while riding a vehicle, all threat goes to the vehicle, not the pilot
+    if (Unit* vehicle = target->GetVehicleBase())
+    {
+        AddThreat(vehicle, amount, spell, ignoreModifiers, ignoreRedirects);
+        if (target->HasUnitTypeMask(UNIT_MASK_ACCESSORY)) // accessories are fully treated as components of the parent and cannot have threat
+            return;
+        amount = 0.0f;
+    }
+
     // if we cannot actually have a threat list, we instead just set combat state and avoid creating threat refs altogether
     if (!CanHaveThreatList())
     {