Adapt Java test to AssistantMessageEventData constructor change

The regenerated AssistantMessageEventData record dropped a field
(now 16 components), but the handwritten test factory still passed
17 positional args, breaking test compilation. Remove the extra
trailing null to match the new constructor arity.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

java/src/test/java/com/github/copilot/SessionEventHandlingTest.java

@@ -865,7 +865,7 @@ private SessionStartEvent createSessionStartEvent(String sessionId) {
     private AssistantMessageEvent createAssistantMessageEvent(String content) {
         var event = new AssistantMessageEvent();
         var data = new AssistantMessageEvent.AssistantMessageEventData(null, null, content, null, null, null, null,
-                null, null, null, null, null, null, null, null, null, null);
+                null, null, null, null, null, null, null, null, null);
         event.setData(data);
         return event;
     }