Reject invalid master and layout indices in view.slideMaster

Found while testing crates/engine/src/cmd/design/master_view.rs:8 (master_view; `master` read via usize_param, `layout` at :14)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/design/master_view.rs

@@ -1,17 +1,17 @@
 use super::{
     Arc, Fill, NO_ID_SPACE, PhType, Result, Run, RunKind, Session, Shape, ShapeId, Size, Target, TextBody, Value, bad, bool_param, json,
-    master_index, ok, opt_usize_param, rect_param, str_param, usize_param,
+    master_index, ok, opt_usize_param, rect_param, str_param,
 };
 
 pub(super) fn master_view(s: &mut Session, p: &Value) -> Result<Value> {
     let st = s.doc()?;
-    let mi = usize_param(p, "master").unwrap_or_else(|| master_index(&st.doc, &st.selection));
+    let mi = opt_usize_param(p, "master", "view.slideMaster")?.unwrap_or_else(|| master_index(&st.doc, &st.selection));
     // A master thumbnail sends `{master}` and means the master itself. `{master, layout}` selects
     // that layout. With neither key, follow the current slide's layout.
     let layout = if p.get("master").is_some() && p.get("layout").is_none() {
         None
     } else {
-        usize_param(p, "layout")
+        opt_usize_param(p, "layout", "view.slideMaster")?
             .or_else(|| st.current_slide().and_then(|sl| st.doc.masters.get(mi).and_then(|m| m.layouts.iter().position(|l| l.id == sl.layout))))
     };
     let target = match layout {

crates/engine/src/tests.rs

@@ -6458,3 +6458,14 @@ fn master_body_placeholder_format_keeps_per_level_formatting() {
     assert_eq!((level(&s, 0).run.size, level(&s, 1).run.size), (Some(28.0), Some(24.0)));
     assert_eq!(shown(&s), vec![Some(28.0), Some(24.0)]);
 }
+
+#[test]
+fn slide_master_view_rejects_invalid_indices() {
+    let mut s = Session::with_new();
+    for p in [json!({"master": -1}), json!({"master": 0.5}), json!({"master": 0, "layout": -1}), json!({"master": 0, "layout": 1.5})] {
+        let r = s.execute("view.slideMaster", &p);
+        assert!(r.is_err(), "invalid index accepted for {p}: {r:?}");
+    }
+    assert!(s.execute("view.slideMaster", &json!({})).is_ok());
+    assert!(s.execute("view.slideMaster", &json!({"master": 0})).is_ok());
+}