[Clarification]: DefaultPushNotificationSender does not set the a2a-version header on outgoing push notifications

Hi, thanks for the great work on the SDK.

When using DefaultPushNotificationSender, outgoing push notifications do not include the a2a-version header. This applies both for v1.0 and when using the legacyCompat serializer with v0.3. The receiver has no way to know which protocol version the body conforms to without inspecting the body itself.

Look for the DefaultPushNotificationSender class in the codebase, likely in a file like src/senders/defaultPushNotificationSender.ts or similar. Examine the method that sends push notifications to see how headers are set. The fix is to add the 'a2a-version' header based on the protocol version being used. Check existing tests for push notifications to understand the expected behavior and add a test for the new header.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.