Core/Auras: implement mana cost modification aura

Port From (https://github.com/TrinityCore/TrinityCore/commit/a76c08843fc1695241b7246c4014322ee07a6453)

Source/Framework/Constants/Spells/SpellAuraConst.cs

@@ -444,7 +444,7 @@ namespace Framework.Constants
         ModBattlePetXpPct = 420,
         ModAbsorbEffectsDonePct = 421, // NYI
         ModAbsorbEffectsTakenPct = 422, //NYI
-        Unk423 = 423,
+        ModManaCostPct = 423,
         CasterIgnoreLos = 424, //NYI
         Unk425 = 425,
         Unk426 = 426,

Source/Game/Entities/StatSystem.cs

@@ -932,6 +932,8 @@ namespace Game.Entities
             return Math.Max(resistMech, 0);
         }
         
+        public void ApplyModManaCostMultiplier(float manaCostMultiplier, bool apply) { ApplyModUpdateFieldValue(m_values.ModifyValue(m_unitData).ModifyValue(m_unitData.ManaCostMultiplier), manaCostMultiplier, apply); }
+
         public void ApplyModManaCostModifier(SpellSchools school, int mod, bool apply) { ApplyModUpdateFieldValue(ref m_values.ModifyValue(m_unitData).ModifyValue(m_unitData.ManaCostModifier, (int)school), mod, apply); }
     }
 

Source/Game/Spells/Auras/AuraEffect.cs

@@ -3461,6 +3461,15 @@ namespace Game.Spells
             }
         }
 
+        [AuraEffectHandler(AuraType.ModManaCostPct)]
+        void HandleModManaCostPct(AuraApplication aurApp, AuraEffectHandleModes mode, bool apply)
+        {
+            if (!mode.HasAnyFlag(AuraEffectHandleModes.ChangeAmountMask | AuraEffectHandleModes.Stat))
+                return;
+
+            aurApp.GetTarget().ApplyModManaCostMultiplier(GetAmount() / 100.0f, apply);
+        }
+        
         [AuraEffectHandler(AuraType.ModPowerDisplay)]
         void HandleAuraModPowerDisplay(AuraApplication aurApp, AuraEffectHandleModes mode, bool apply)
         {

Source/Game/Spells/SpellInfo.cs

@@ -2773,9 +2773,6 @@ namespace Game.Spells
                         flatMod += eff.GetAmount();
                     }
 
-                    if (power.PowerType == PowerType.Mana)
-                        flatMod *= (int)(1.0f + caster.m_unitData.ManaCostMultiplier); // this is wrong
-
                     powerCost += flatMod;
                 }
 
@@ -2842,7 +2839,9 @@ namespace Game.Spells
                     powerCost += MathFunctions.CalculatePct(powerCost, eff.GetAmount());
                 }
 
-                if (power.PowerType == PowerType.Health)
+                if (power.PowerType == PowerType.Mana)
+                    powerCost = (int)((float)powerCost * (1.0f + caster.m_unitData.ManaCostMultiplier));
+                else if (power.PowerType == PowerType.Health)
                 {
                     healthCost += powerCost;
                     continue;