Validate ShaderPrototype IDs (#38728)

* Convert all shader prototype string literals to protoids in overlays

* Convert more shader prototype literal strings to protoids

* Convert ValidatePrototypeId to ProtoId

* Later

Content.Client/Mapping/MappingOverlay.cs

@@ -11,6 +11,8 @@ namespace Content.Client.Mapping;
 
 public sealed class MappingOverlay : Overlay
 {
+    private static readonly ProtoId<ShaderPrototype> UnshadedShader = "unshaded";
+
     [Dependency] private readonly IEntityManager _entities = default!;
     [Dependency] private readonly IPlayerManager _player = default!;
     [Dependency] private readonly IPrototypeManager _prototypes = default!;
@@ -35,7 +37,7 @@ public sealed class MappingOverlay : Overlay
         _sprite = _entities.System<SpriteSystem>();
 
         _state = state;
-        _shader = _prototypes.Index<ShaderPrototype>("unshaded").Instance();
+        _shader = _prototypes.Index(UnshadedShader).Instance();
     }
 
     protected override void Draw(in OverlayDrawArgs args)