PDF export: an image that can't be decoded is left out with a note instead of failing the export

When the PDF writer keeps a PNG, GIF or WebP image as it is, it accepts
the image once its header reads, and it reports pixel data that it
can't decode only when it finishes the file. An image whose pixel data
was cut short or damaged therefore failed the whole export with "PDF
writer error: Image(...)", and nothing was written. This happened in
Save as PDF, .ai saves, Save a Copy, Print, Export for Screens and
native files with PDF compatibility, and with On Copy: PDF turned on,
Copy left the PDF off the clipboard.

When the writer fails on an image's pixel data, the export now runs
again with that image left out. Its warnings hold the note that raster
exports give, once: "an image couldn't be decoded and is missing from
the picture". This note replaces the writer's "image '<key>' could not
be decoded and was skipped", which named the image by an internal key.
An export without such an image runs once, as before. A .ai file keeps
the image in its editing data and reopens with it.

Docs: docs/mcp.md (PDF options).

crates/engine/src/cmd/fileio/tests_pdf.rs

@@ -131,6 +131,56 @@ fn options_not_applied_yet_and_document_features_warn() {
     assert!(warnings(&s.execute("document.export", &json!({"format": "png"})).unwrap()).is_empty());
 }
 
+/// A placed image whose pixel data can't be decoded is left out of every PDF the engine writes,
+/// and the rest of the page is written. Each reply that carries the PDF's warnings has the note
+/// raster exports give, once.
+#[test]
+fn every_pdf_leaves_out_an_image_that_cant_be_decoded() {
+    const NOTE: &str = "an image couldn't be decoded and is missing from the picture";
+    let mut s = session(1);
+    let cut = crate::tests_menucmds::place_undecodable_png(&mut s);
+    // Whether each object on the PDF's pages is an image. The pages should hold the rectangle and
+    // no image.
+    let drawn = |bytes: &[u8]| {
+        let mut images = vec![];
+        vectorcraft_pdf::import(bytes).unwrap().walk(|n| {
+            if !n.is_container() && !matches!(n.kind, vectorcraft_doc::NodeKind::Path { clipping: true, .. }) {
+                images.push(matches!(n.kind, vectorcraft_doc::NodeKind::Image(_)));
+            }
+        });
+        images
+    };
+    for (cmd, p) in [
+        ("document.exportPdf", json!({})),
+        ("document.exportPdf", json!({"preset": "High Quality Print"})),
+        ("document.exportPdf", json!({"standard": "pdfX1a"})),
+        ("document.export", json!({"format": "pdf"})),
+        ("file.saveCopy", json!({"format": "pdf"})),
+        ("file.saveCopy", json!({"format": "ai"})),
+        ("file.print", json!({})),
+    ] {
+        let v = s.execute(cmd, &p).unwrap_or_else(|e| panic!("{cmd} {p}: {e}"));
+        assert_eq!(warnings(&v).iter().filter(|w| *w == NOTE).count(), 1, "{cmd} {p}: {}", v["warnings"]);
+        assert_eq!(drawn(&b64(&v)), [false], "{cmd} {p}");
+    }
+    // The .ai file reopens with the image as it was placed, from the image bytes in its editing data.
+    let ai = b64(&s.execute("file.saveCopy", &json!({"format": "ai"})).unwrap());
+    let placed = s.doc().unwrap().doc.images.clone();
+    s.execute("document.open", &json!({"name": "cut.ai", "dataBase64": vectorcraft_format::base64_encode(&ai)})).unwrap();
+    let reopened = &s.doc().unwrap().doc;
+    assert!(reopened.node(cut).is_some_and(|n| matches!(n.kind, vectorcraft_doc::NodeKind::Image(_))));
+    assert_eq!(reopened.images, placed);
+    // Export for Screens, a native file carrying a PDF, and the PDF that Copy offers are written.
+    let r = s.execute("document.exportForScreens", &json!({"formats": [{"format": "pdf"}]})).unwrap();
+    assert_eq!(drawn(&b64(&r["files"][0])), [false]);
+    assert!(s.execute("file.saveCopy", &json!({"format": "vectorcraft", "pdfCompatible": true})).is_ok());
+    s.execute("select.set", &json!({"ids": [cut.0]})).unwrap();
+    s.execute("edit.copy", &json!({})).unwrap();
+    assert!(b64(&s.execute("clipboard.exportPdf", &json!({})).unwrap()).starts_with(b"%PDF"));
+    s.execute("prefs.set", &json!({"key": "copyAsPdf", "value": true})).unwrap();
+    assert!(s.clipboard_flavours().iter().any(|f| f.mime == crate::cmd::clipboard::PDF));
+}
+
 #[test]
 fn export_pdf_writes_a_path() {
     let mut s = session(2);