Forbid string literals for NpcFactionSystem methods (#38140)

* Add ForbidLiteral attribute to NpcFactionSystem methods

* Cleanup resulting warnings

Content.Server/Zombies/ZombieSystem.Transform.cs

@@ -38,6 +38,7 @@ using Content.Shared.Ghost.Roles.Components;
 using Content.Shared.Tag;
 using Robust.Shared.Player;
 using Robust.Shared.Prototypes;
+using Content.Shared.NPC.Prototypes;
 
 namespace Content.Server.Zombies;
 
@@ -66,6 +67,8 @@ public sealed partial class ZombieSystem
 
     private static readonly ProtoId<TagPrototype> InvalidForGlobalSpawnSpellTag = "InvalidForGlobalSpawnSpell";
     private static readonly ProtoId<TagPrototype> CannotSuicideTag = "CannotSuicide";
+    private static readonly ProtoId<NpcFactionPrototype> ZombieFaction = "Zombie";
+
     /// <summary>
     /// Handles an entity turning into a zombie when they die or go into crit
     /// </summary>
@@ -223,7 +226,7 @@ public sealed partial class ZombieSystem
         _mobState.ChangeMobState(target, MobState.Alive);
 
         _faction.ClearFactions(target, dirty: false);
-        _faction.AddFaction(target, "Zombie");
+        _faction.AddFaction(target, ZombieFaction);
 
         //gives it the funny "Zombie ___" name.
         _nameMod.RefreshNameModifiers(target);