fix(mission-server): Fix GetItemsSnapshot return type

Changed the return type of GetItemsSnapshot to ImmutableArray. This ensures the returned collection is immutable, providing better security and stability.

src/Asv.Mavlink/Microservices/Missions/Server/Ex/MissionServerEx.cs

@@ -138,7 +138,7 @@ public void RemoveItems(IEnumerable<ServerMissionItem> items)
 
     public ImmutableArray<ServerMissionItem> GetItemsSnapshot()
     {
-        return [.._missionSource.Items];
+        return  _missionSource.Items.ToImmutableArray();
     }
 }