Fix up merged PRs: no debug_assert in blank_presentation; more exhaustion coverage

blank_presentation's debug_assert would panic in debug builds (never-crash
rule): a fresh deck that cannot fit its title slide now simply arrives
without it. The exhaustion test also covers insert.smartArt and
insert.textBox.

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

crates/engine/src/tests.rs

@@ -3469,6 +3469,8 @@ fn an_exhausted_id_space_refuses_growth() {
     assert!(s.execute("shape.insert", &json!({"preset": "rect", "rect": [10, 10, 100, 50]})).is_err());
     assert!(s.execute("slide.duplicate", &json!({})).is_err());
     assert!(s.execute("edit.duplicate", &json!({})).is_err());
+    assert!(s.execute("insert.smartArt", &json!({"kind": "blockList", "items": ["Plan", "Build"]})).is_err());
+    assert!(s.execute("insert.textBox", &json!({"text": "Hi"})).is_err());
     // What the deck holds is untouched: same slides, same ids, no duplicates.
     let after = s.doc().unwrap().doc.clone();
     assert_eq!(after.slides.len(), before.slides.len());

crates/model/src/defaults.rs

@@ -522,9 +522,9 @@ pub fn blank_presentation(size: Size, theme: Theme, title_slide: bool) -> Presen
         // A fresh deck starts at id 256 and this builds a handful of ids, so the slide always
         // fits; should that ever stop holding, the deck arrives without its title slide rather
         // than with a duplicated id.
-        let s = new_slide(&mut p, first_layout);
-        debug_assert!(s.is_some(), "a fresh deck cannot run out of ids");
-        p.slides.extend(s.map(|s| vec![Arc::new(s)]).unwrap_or_default());
+        if let Some(s) = new_slide(&mut p, first_layout) {
+            p.slides.push(Arc::new(s));
+        }
     }
     p
 }