file.export: a perPage that isn't a count is refused, not read as 6

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

crates/engine/src/cmd/file.rs

@@ -360,7 +360,7 @@ pub fn pdf_options(p: &Value, count: usize) -> Result<deckcraft_pdf::PdfOptions>
     o.layout = match str_param(p, "layout").unwrap_or("slides") {
         "notes" => deckcraft_pdf::PageLayout::Notes,
         "handouts" => {
-            let per_page = usize_param(p, "perPage").unwrap_or(6);
+            let per_page = opt_usize_param(p, "perPage", "file.export")?.unwrap_or(6);
             if ![1, 2, 3, 4, 6, 9].contains(&per_page) {
                 return Err(bad("file.export", format!("perPage {per_page} isn't one of 1, 2, 3, 4, 6, 9")));
             }

crates/engine/src/tests.rs

@@ -2421,6 +2421,10 @@ fn pdf_options_reject_unsupported_per_page() {
         let e = cmd::file::pdf_options(&json!({"layout": "handouts", "perPage": n}), 1).unwrap_err();
         assert!(e.to_string().contains("1, 2, 3, 4, 6, 9"), "perPage {n}: {e}");
     }
+    // Not a count at all: refused too, instead of falling back to 6.
+    for v in [json!(-1), json!(5.5), json!("6")] {
+        assert!(cmd::file::pdf_options(&json!({"layout": "handouts", "perPage": v}), 1).is_err(), "perPage {v}");
+    }
 }
 
 #[test]