examples: fix using_serve_dir_with_assets_fallback to not expose assets at root (#3762)

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

examples/static-file-server/src/tests.rs

@@ -0,0 +1,28 @@
+use super::using_serve_dir_with_assets_fallback;
+use axum::{body::Body, http::Request, http::StatusCode};
+use http_body_util::BodyExt;
+use tower::ServiceExt;
+
+const INDEX_HTML_CONTENT: &str = include_str!("../assets/index.html");
+
+// `/script.js` at the root is not an asset under `/assets`, so it falls through to the SPA
+// fallback. The fallback serves `index.html` with a 404 status -- the path wasn't found, but
+// the SPA handles routing client-side.
+#[tokio::test]
+async fn assets_not_served_at_root() {
+    let app = using_serve_dir_with_assets_fallback();
+    let response = app
+        .oneshot(
+            Request::builder()
+                .uri("/script.js")
+                .body(Body::empty())
+                .unwrap(),
+        )
+        .await
+        .unwrap();
+
+    assert_eq!(response.status(), StatusCode::NOT_FOUND);
+    let body = response.into_body().collect().await.unwrap().to_bytes();
+    let body_str = String::from_utf8(body.to_vec()).unwrap();
+    assert_eq!(body_str, INDEX_HTML_CONTENT);
+}