fix(test): give the synthetic plot image the new pixelated field

#1488 added ImageModel::pixelated; the PDF export image check builds one
by hand and no longer compiled.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/pdf_export_images_check.rs

@@ -145,6 +145,7 @@ fn image() -> PlotImage {
                     uv: uv[i],
                 })
                 .collect(),
+            pixelated: true,
         },
         clips: Vec::new(),
     }