test(js): cover multipart request shape

Cover multipart runtime request construction so file-upload forms keep FormData, omit File values from URL encoding, and only mark URLSearchParams bodies as urlencoded.

req: multipart/003

slhx-js/tests/runtime.rs

@@ -9,6 +9,17 @@ fn runtime_posts_urlencoded_forms_by_default() {
     assert!(source.contains("application/x-www-form-urlencoded;charset=UTF-8"));
 }
 
+#[test]
+fn runtime_preserves_multipart_file_upload_fallback_shape() {
+    // req: multipart/003
+    let source = slhx_js::RUNTIME_JS;
+
+    assert!(source.contains("multipart: form && String(form.enctype).toLowerCase() === \"multipart/form-data\""));
+    assert!(source.contains("if (value instanceof File) continue"));
+    assert!(source.contains("return multipart ? data : urlEncoded(data)"));
+    assert!(source.contains("if (body instanceof URLSearchParams) headers[\"Content-Type\"] = \"application/x-www-form-urlencoded;charset=UTF-8\""));
+}
+
 #[test]
 fn runtime_interval_dispatch_avoids_duplicate_timers() {
     // req: convention/005