Lint more const string prototypes (#18922)

Content.Server/GameTicking/GameTicker.Spawning.cs

@@ -9,13 +9,13 @@ using Content.Server.Speech.Components;
 using Content.Server.Station.Components;
 using Content.Shared.CCVar;
 using Content.Shared.Database;
-using Content.Shared.GameTicking;
 using Content.Shared.Preferences;
 using Content.Shared.Roles;
 using JetBrains.Annotations;
 using Robust.Server.Player;
 using Robust.Shared.Map;
 using Robust.Shared.Network;
+using Robust.Shared.Prototypes;
 using Robust.Shared.Random;
 using Robust.Shared.Utility;
 using Job = Content.Server.Roles.Job;
@@ -26,6 +26,7 @@ namespace Content.Server.GameTicking
     {
         [Dependency] private readonly IAdminManager _adminManager = default!;
 
+        [ValidatePrototypeId<EntityPrototype>]
         private const string ObserverPrototypeName = "MobObserver";
 
         /// <summary>