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/Light/SunShadowOverlay.cs

@@ -11,6 +11,8 @@ namespace Content.Client.Light;
 
 public sealed class SunShadowOverlay : Overlay
 {
+    private static readonly ProtoId<ShaderPrototype> MixShader = "Mix";
+
     public override OverlaySpace Space => OverlaySpace.BeforeLighting;
 
     [Dependency] private readonly IClyde _clyde = default!;
@@ -150,7 +152,7 @@ public sealed class SunShadowOverlay : Overlay
                         viewport.LightRenderTarget.GetWorldToLocalMatrix(eye, scale);
                     worldHandle.SetTransform(invMatrix);
 
-                    var maskShader = _protoManager.Index<ShaderPrototype>("Mix").Instance();
+                    var maskShader = _protoManager.Index(MixShader).Instance();
                     worldHandle.UseShader(maskShader);
 
                     worldHandle.DrawTextureRect(_target.Texture, worldBounds, Color.Black.WithAlpha(alpha));