Fix Java close cancellation test after detach migration

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

Author: stephentoub

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

@@ -108,6 +108,10 @@ void testExternalToolCompletedCancelsBlockedHandler() throws Exception {
 
     @Test
     void testCloseCancelsBlockedExternalTool() throws Exception {
+        var rpc = mock(JsonRpcClient.class);
+        when(rpc.invoke(eq("session.detach"), any(), eq(CopilotSession.SessionDetachResponse.class)))
+                .thenReturn(CompletableFuture.completedFuture(new CopilotSession.SessionDetachResponse(true, null)));
+        session = createTestSession(rpc);
         var toolFuture = new CompletableFuture<Object>();
         var started = new CountDownLatch(1);
         session.registerTools(List.of(ToolDefinition.create("blocked_tool", "Blocks", Map.of(), invocation -> {