test(pdf-export): build ImageModel with use_alpha

#1513 added the field; this integration test's struct literal was not updated, so the test target no longer compiled.

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

tests/pdf_export_images_check.rs

@@ -146,6 +146,7 @@ fn image() -> PlotImage {
                 })
                 .collect(),
             pixelated: true,
+            use_alpha: true,
         },
         clips: Vec::new(),
     }