Add missing unidentified keep-alive methods to test stub

Implement addUnidentifiedKeepAlive and removeUnidentifiedKeepAlive in SseInitialFlushTest's Manager stub to match the updated interface.

src/test/java/org/asamk/signal/http/SseInitialFlushTest.java

@@ -447,6 +447,14 @@ class SseInitialFlushTest {
         public void addClosedListener(Runnable listener) {
         }
 
+        @Override
+        public void addUnidentifiedKeepAlive(String token) {
+        }
+
+        @Override
+        public void removeUnidentifiedKeepAlive(String token) {
+        }
+
         @Override
         public InputStream retrieveAttachment(String id) {
             return null;