Core/Players/DK: Implement Rune Grace Period mechanic

Closes #11736
Fixes #6122

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

@@ -7604,7 +7604,7 @@ bool Unit::HandleAuraProc(Unit* victim, uint32 damage, Aura* triggeredByAura, Sp
                                 player->GetBaseRune(i) != RUNE_BLOOD)
                                 continue;
                         }
-                        if (player->GetRuneCooldown(i) != player->GetRuneBaseCooldown(i))
+                        if (player->GetRuneCooldown(i) != (player->GetRuneBaseCooldown(i) - player->GetLastRuneGraceTimer(i)))
                             continue;
 
                         --runesLeft;
@@ -11760,6 +11760,16 @@ void Unit::ClearInCombat()
     m_CombatTimer = 0;
     RemoveFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_IN_COMBAT);
 
+    // Reset rune flags after combat
+    if (GetTypeId() == TYPEID_PLAYER && getClass() == CLASS_DEATH_KNIGHT)
+    {
+        for (uint8 i = 0; i < MAX_RUNES; ++i)
+        {
+            ToPlayer()->SetRuneTimer(i, 0xFFFFFFFF);
+            ToPlayer()->SetLastRuneGraceTimer(i, 0);
+        }
+    }
+
     // Player's state will be cleared in Player::UpdateContestedPvP
     if (Creature* creature = ToCreature())
     {