Integration: prove doc.replaceSmartObject replaces and undoes (#8150)

The rpc test only checked the returned layer id, so a no-op that echoed
{"layer": id} passed, its undo check passed on doc.place's own step, and
the bad-layer case asserted on an empty reason. It now checks the smart
object's file name and rendered centre pixel (42 -> 200 after the
replace, back to 42 on undo, 200 on redo), that every refusal changes
nothing, and that layer 999999 fails with "no such layer".

The bridge row in docs/control-protocol.md claimed doc_replace_smart_object
maps onto layer.smartObjects.replaceContents; the desktop refuses that
command (ambient path), like doc_place's file.placeEmbedded. The row now
says so and points to the headless / workspace server method.

Co-authored-by: VishnuByrraju <112673081+VishnuByrraju@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/automation/src/rpc.rs

@@ -712,22 +712,42 @@ mod tests {
         let mut h = Headless::with_workspace(crate::AuthorizedWorkspace::new(Some(&root), Some(&root)).unwrap());
         h.handle("doc.new", json!({"width": 40, "height": 30})).unwrap();
         let layer = h.handle("doc.place", json!({"path": "logo.png"})).unwrap()["layer"].clone();
+        // The smart object's file name and the composite at its centre: what Replace Contents changes.
+        let contents = |h: &mut Headless| {
+            let inspected = h.handle("doc.inspect", json!({})).unwrap();
+            let layers = inspected["layers"].as_array().unwrap();
+            let so = layers.iter().find(|l| l["id"] == layer).unwrap_or_else(|| panic!("{inspected}"));
+            let name = so["smartSource"]["fileName"].as_str().unwrap_or_default().to_owned();
+            let rendered = h.handle("doc.render", json!({"maxSide": 0})).unwrap();
+            let png = base64::engine::general_purpose::STANDARD.decode(rendered["base64"].as_str().unwrap()).unwrap();
+            let img = photocraft_codecs::decode(&png).unwrap();
+            let (w, ht) = img.dimensions();
+            let data = img.as_u8().unwrap();
+            let channels = data.len() / (w * ht) as usize;
+            (name, data[((ht / 2 * w + w / 2) as usize) * channels])
+        };
+        assert_eq!(contents(&mut h), ("logo.png".to_owned(), 42), "the placed contents");
         let replaced = h.handle("doc.replaceSmartObject", json!({"path": "red.png", "layer": layer})).unwrap();
         assert_eq!(replaced["layer"], layer, "{replaced}");
-        let undo = h.handle("engine.execute", json!({"command": "edit.undo"}));
-        assert!(undo.is_ok(), "the replacement is one history step: {undo:?}");
+        assert_eq!(contents(&mut h), ("red.png".to_owned(), 200), "the contents were replaced");
+        h.handle("engine.execute", json!({"command": "edit.undo"})).unwrap();
+        assert_eq!(contents(&mut h), ("logo.png".to_owned(), 42), "one undo step restores the old contents");
+        h.handle("engine.execute", json!({"command": "edit.redo"})).unwrap();
+        assert_eq!(contents(&mut h), ("red.png".to_owned(), 200), "redo replaces them again");
         // Outside the read root, a bad layer, a missing path or a file that is no image: refused, nothing changed.
         for (params, reason) in [
             (json!({"path": "../outside/red.png"}), "parent traversal"),
             (json!({"path": absolute}), "must be inside the automation root"),
-            (json!({"path": "red.png", "layer": 999_999}), ""),
+            (json!({"path": "logo.png", "layer": 999_999}), "no such layer"),
             (json!({}), "needs `path`"),
         ] {
             let refused = h.handle("doc.replaceSmartObject", params).unwrap_err().to_string();
             assert!(refused.contains(reason), "{refused}");
+            assert_eq!(contents(&mut h), ("red.png".to_owned(), 200), "nothing changed: {refused}");
         }
         std::fs::write(root.join("junk.png"), b"not an image").unwrap();
         assert!(h.handle("doc.replaceSmartObject", json!({"path": "junk.png", "layer": layer})).is_err());
+        assert_eq!(contents(&mut h), ("red.png".to_owned(), 200), "a file that is no image changes nothing");
         // The ambient command keeps its refusal.
         let ambient = h.handle("engine.execute", json!({"command": "layer.smartObjects.replaceContents", "params": {"path": "red.png"}}));
         assert!(ambient.unwrap_err().to_string().contains("ambient filesystem paths"));