test(axum): close page edge-case mutants

Prove partial constructor defaults, missing multipart-boundary rejection, first-root fingerprint replacement, and unchanged non-root HTML through public and nearest-boundary paths.

req: page_swap/001

req: multipart/003

req: abi/005

hemx-axum/src/lib.rs

@@ -1609,6 +1609,18 @@ mod tests {
             .expect("root element is rendered");
         assert_eq!(root.value().attr("data-hemx-fp"), Some("1"));
         assert_eq!(root.text().collect::<String>(), "Docs");
+        assert_eq!(html.matches("data-hemx-fp=").count(), 1);
+        assert!(!html.contains("data-hemx-fp=\"99\""));
+    }
+
+    #[test]
+    fn fingerprint_injection_leaves_html_without_a_hemx_root_unchanged() {
+        let html = "<main>Docs</main>".to_owned();
+        assert_eq!(
+            html_with_root_fingerprint(html.clone(), BuildFingerprint(99)),
+            html
+        );
+        // test req: abi/005
     }
 
     #[test]