fix: Add Content-Type header to push notification requests

Set `Content-Type: application/json` header when sending push notifications
to prevent `415 Unsupported Media Type` errors. The request was previously
defaulting to `application/octet-stream`.

Updates tests to verify that the `Content-Type` header is always present
in push notification requests, regardless of whether authentication
token is included.

Fixes: #486
Signed-off-by: Jeff Mesnil <[email protected]>

Author: jmesnil

server-common/src/main/java/io/a2a/server/tasks/BasePushNotificationSender.java

@@ -90,6 +90,7 @@ private boolean dispatchNotification(Task task, PushNotificationConfig pushInfo)
         try {
             postBuilder
                     .url(url)
+                    .addHeader("Content-Type", "application/json")
                     .body(body)
                     .post();
         } catch (IOException | InterruptedException e) {

server-common/src/test/java/io/a2a/server/tasks/InMemoryPushNotificationConfigStoreTest.java

@@ -50,6 +50,7 @@ public void setUp() {
     private void setupBasicMockHttpResponse() throws Exception {
         when(mockHttpClient.createPost()).thenReturn(mockPostBuilder);
         when(mockPostBuilder.url(any(String.class))).thenReturn(mockPostBuilder);
+        when(mockPostBuilder.addHeader("Content-Type", "application/json")).thenReturn(mockPostBuilder);
         when(mockPostBuilder.body(any(String.class))).thenReturn(mockPostBuilder);
         when(mockPostBuilder.post()).thenReturn(mockHttpResponse);
         when(mockHttpResponse.success()).thenReturn(true);
@@ -233,6 +234,7 @@ public void testSendNotificationSuccess() throws Exception {
         when(mockHttpClient.createPost()).thenReturn(mockPostBuilder);
         when(mockPostBuilder.url(any(String.class))).thenReturn(mockPostBuilder);
         when(mockPostBuilder.body(any(String.class))).thenReturn(mockPostBuilder);
+        when(mockPostBuilder.addHeader("Content-Type", "application/json")).thenReturn(mockPostBuilder);
         when(mockPostBuilder.post()).thenReturn(mockHttpResponse);
         when(mockHttpResponse.success()).thenReturn(true);
 

server-common/src/test/java/io/a2a/server/tasks/PushNotificationSenderTest.java

@@ -1,6 +1,7 @@
 package io.a2a.server.tasks;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.io.IOException;
@@ -158,7 +159,11 @@ private void testSendNotificationWithInvalidToken(String token, String testName)
         // Verify that no authentication header was sent (invalid token should not add header)
         assertEquals(1, testHttpClient.headers.size());
         Map<String, String> sentHeaders = testHttpClient.headers.get(0);
-        assertTrue(sentHeaders.isEmpty(), "No headers should be sent when token is invalid");
+        assertFalse(sentHeaders.containsKey(A2AHeaders.X_A2A_NOTIFICATION_TOKEN),
+                "X-A2A-Notification-Token header should not be sent when token is invalid");
+        // Content-Type header should always be present
+        assertTrue(sentHeaders.containsKey("Content-Type"), "Content-Type header should be present");
+        assertEquals("application/json", sentHeaders.get("Content-Type"));
     }
 
     private Task createSampleTask(String taskId, TaskState state) {
@@ -229,6 +234,10 @@ public void testSendNotificationWithTokenSuccess() throws InterruptedException {
         Map<String, String> sentHeaders = testHttpClient.headers.get(0);
         assertTrue(sentHeaders.containsKey(A2AHeaders.X_A2A_NOTIFICATION_TOKEN));
         assertEquals(config.token(), sentHeaders.get(A2AHeaders.X_A2A_NOTIFICATION_TOKEN));
+        // Content-Type header should always be present
+        assertTrue(sentHeaders.containsKey("Content-Type"), "Content-Type header should be present");
+        assertEquals("application/json", sentHeaders.get("Content-Type"));
+
     }
 
     @Test
@@ -290,10 +299,10 @@ public void testSendNotificationHttpError() {
         String taskId = "task_send_http_err";
         Task taskData = createSampleTask(taskId, TaskState.COMPLETED);
         PushNotificationConfig config = createSamplePushConfig("http://notify.me/http_error", "cfg1", null);
-        
+
         // Set up the configuration in the store
         configStore.setInfo(taskId, config);
-        
+
         // Configure the test client to throw an exception
         testHttpClient.shouldThrowException = true;