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/tests/response.rs

@@ -144,6 +144,19 @@ async fn interaction_boundary_skips_unnamed_parts_and_rejects_invalid_multipart(
     let _guard = BOUNDARY_TEST_LOCK
         .lock()
         .unwrap_or_else(|error| error.into_inner());
+    let app = Router::new().route("/mutate", post(bounded_mutation));
+    let missing_boundary = app
+        .clone()
+        .oneshot(
+            Request::post("/mutate")
+                .header(header::CONTENT_TYPE, "multipart/form-data")
+                .body(Body::from("invalid"))
+                .unwrap(),
+        )
+        .await
+        .unwrap();
+    assert_eq!(missing_boundary.status(), StatusCode::BAD_REQUEST);
+
     let accepted = concat!(
         "--b\r\n",
         "Content-Disposition: form-data; filename=\"ignored.txt\"\r\n\r\n",
@@ -153,7 +166,6 @@ async fn interaction_boundary_skips_unnamed_parts_and_rejects_invalid_multipart(
         "1\r\n",
         "--b--\r\n"
     );
-    let app = Router::new().route("/mutate", post(bounded_mutation));
     let response = app
         .clone()
         .oneshot(
@@ -369,6 +381,7 @@ fn page_response_constructors_preserve_mode_and_optional_fingerprint() {
     let partial = PageResponse::partial("<main>Partial</main>");
     assert_eq!(partial.mode, PageMode::Partial);
     assert_eq!(partial.html, "<main>Partial</main>");
+    assert_eq!(partial.title, None);
     assert_eq!(partial.fingerprint, None);
 
     let fingerprint = BuildFingerprint(42);