Add the missing XHR JSON echo fixture

examples/runtime/xhr/src/xhr.js

@@ -260,7 +260,7 @@ export function postWithFormData(port) {
 export function postWithJsonBody(port) {
     console.log('XMLHttpRequest test 12: POST with JSON body');
 
-    return new Promise((resolve) => {
+    return new Promise((resolve, reject) => {
         const xhr = new XMLHttpRequest();
 
         const jsonData = {
@@ -271,8 +271,29 @@ export function postWithJsonBody(port) {
         xhr.onload = function() {
             console.log(`JSON POST status: ${xhr.status}`);
             console.log(`JSON response: ${xhr.response}`);
+
+            try {
+                const response = JSON.parse(xhr.response);
+                if (
+                    xhr.status !== 200 ||
+                    response.name !== jsonData.name ||
+                    response.value !== jsonData.value
+                ) {
+                    reject(new Error('JSON echo response did not match the request'));
+                    return;
+                }
+            } catch (error) {
+                reject(error);
+                return;
+            }
+
+            console.log('JSON response matched request: true');
             resolve();
         };
+
+        xhr.onerror = function() {
+            reject(new Error('JSON echo request failed'));
+        };
 
         xhr.open('POST', `http://localhost:${port}/json-echo`);
         xhr.setRequestHeader('Content-Type', 'application/json');

tests/common/test_server.rs

@@ -187,6 +187,14 @@ pub async fn start_test_server() -> (u16, TestServerHandle) {
                     }))
                 }),
             )
+            .route(
+                "/json-echo",
+                post(async move |Json(body): Json<serde_json::Value>| {
+                    // For accepted JSON requests, extraction drains and validates the body before
+                    // this handler builds the response.
+                    Json(body)
+                }),
+            )
             .route(
                 "/echo-referer",
                 post(async move |headers: HeaderMap, _body: Bytes| {

tests/runtime/xhr.rs

@@ -245,7 +245,8 @@ async fn xhr_post_with_json_body(
     println!("{output}");
 
     assert!(output.contains("XMLHttpRequest test 12: POST with JSON body"));
-    assert!(output.contains("JSON POST status:") || output.contains("ReadyState changed"));
+    assert!(output.contains("JSON POST status: 200"));
+    assert!(output.contains("JSON response matched request: true"));
 
     Ok(())
 }