Slides: going to a slide that does not exist is an error

slide.go clamped an out-of-range index to the last slide and reported
success, so a request for slide 99 of a short deck landed on the last slide
without a word. It now returns an error and stays on the current slide. A slide
id beyond u32 is reported as an unknown slide instead of being read as id 0.

crates/engine/src/cmd/slide.rs

@@ -283,9 +283,16 @@ fn go_to(s: &mut Session, i: usize) -> Result<Value> {
 fn go(s: &mut Session, p: &Value) -> Result<Value> {
     let i = match (usize_param(p, "index"), p.get("id").and_then(Value::as_u64)) {
         (Some(i), _) => i,
-        (None, Some(id)) => s.doc()?.doc.slide_index(SlideId(u32::try_from(id).unwrap_or(0))).ok_or_else(|| bad("slide.go", "no such slide"))?,
+        (None, Some(id)) => s
+            .doc()?
+            .doc
+            .slide_index(SlideId(u32::try_from(id).map_err(|_| bad("slide.go", "no such slide"))?))
+            .ok_or_else(|| bad("slide.go", "no such slide"))?,
         _ => return Err(bad("slide.go", "missing `index`")),
     };
+    if i >= s.doc()?.doc.slides.len() {
+        return Err(bad("slide.go", format!("no slide {i}")));
+    }
     go_to(s, i)
 }
 fn next(s: &mut Session, _p: &Value) -> Result<Value> {

crates/mcp/src/tests.rs

@@ -90,6 +90,26 @@ fn errors_are_reported_not_panics() {
     assert_eq!(r["result"]["isError"], true);
 }
 
+fn current_slide_title(s: &mut Server) -> Value {
+    tool(s, "inspect_slide", json!({}))["title"].clone()
+}
+
+#[test]
+fn go_to_slide_out_of_range_is_error() {
+    let mut s = Server::new(Box::new(Headless::new()));
+    tool(&mut s, "new_presentation", json!({"theme": "Harbor"}));
+    tool(&mut s, "add_slide", json!({"layout": "titleAndContent", "title": "Second", "body": "x"}));
+    // Boundaries stay valid: first and last slide.
+    tool(&mut s, "go_to_slide", json!({"index": 0}));
+    tool(&mut s, "go_to_slide", json!({"index": 1}));
+    assert_eq!(current_slide_title(&mut s), "Second");
+    for bad in [json!(2), json!(99)] {
+        let r = call(&mut s, 1, "tools/call", json!({"name": "go_to_slide", "arguments": {"index": bad}}));
+        assert_eq!(r["result"]["isError"], true, "{bad}: {r}");
+        assert_eq!(current_slide_title(&mut s), "Second", "failed go_to_slide must not move");
+    }
+}
+
 #[test]
 fn export_tool_lists_pdf_and_its_options() {
     let mut s = Server::new(Box::new(Headless::new()));