fix: Allow POST requests without Content-Type header when body is empty (#759)

Per RFC 9110 §8.3, Content-Type header is only meaningful when a message
body is present. The HTTP+JSON transport was incorrectly returning 415
(ContentTypeNotSupportedError) for POST requests with no body and no
Content-Type header.

This fix adds validateContentTypeForOptionalBody() method to handle
endpoints like POST /tasks/{id}:cancel where the body is optional. When
the body is null or empty, Content-Type validation is skipped. When body
content is present, Content-Type must still be application/json.

This allows bodyless POST requests to be processed correctly and return
appropriate errors (404 TaskNotFoundError, 409 TaskNotCancelableError)
instead of 415.

Fixes #747 🦕

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

reference/rest/src/main/java/io/a2a/server/rest/quarkus/A2AServerRoutes.java

@@ -346,7 +346,7 @@ public void getTask(RoutingContext rc) {
      */
     @Route(regex = "^\\/(?<tenant>[^\\/]*\\/?)tasks\\/(?<taskId>[^/]+):cancel$", order = 1, methods = {Route.HttpMethod.POST}, type = Route.HandlerType.BLOCKING)
     public void cancelTask(@Body String body, RoutingContext rc) {
-        if (!validateContentType(rc)) {
+        if (!validateContentTypeForOptionalBody(rc, body)) {
             return;
         }
         String taskId = rc.pathParam("taskId");
@@ -624,6 +624,25 @@ private boolean validateContentType(RoutingContext rc) {
         return true;
     }
 
+    /**
+     * Check if the request content type is application/json when body content is present.
+     * Per RFC 9110 §8.3, Content-Type is only meaningful when a message body is present.
+     * Use this for endpoints where the body is optional.
+     *
+     * @param rc the routing context
+     * @param body the request body (may be null or empty)
+     * @return true if validation passes, false if Content-Type error should be returned
+     */
+    private boolean validateContentTypeForOptionalBody(RoutingContext rc, @Nullable String body) {
+        // If body is null or empty, Content-Type is not required
+        if (body == null || body.isBlank()) {
+            return true;
+        }
+
+        // Body has content - validate Content-Type
+        return validateContentType(rc);
+    }
+
     /**
      * Retrieves the public agent card for agent discovery.
      *