fix(bigtable): resolve data race in ClientTest

Use CopyOnWriteArrayList instead of ArrayList in FakeBigtableService mock
implementation. The mock service receives gRPC requests on gRPC executor
threads and adds them to openSessionRequests and vrpcRequests. The test
thread reads these lists to verify client payloads. Without proper thread
synchronization, visibility and ordering issues can cause the test thread
to read a null element during ArrayList.add, resulting in a NullPointerException
in testMaterializedViewRequestSent.

BUG=b/481669998
TAG=agy
CONV=7b193e4d-38f8-450c-8475-0b9ae0d04507

Author: lqiu96

java-bigtable/google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/internal/api/ClientTest.java

@@ -45,9 +45,9 @@
 import io.grpc.Server;
 import io.grpc.stub.StreamObserver;
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
@@ -257,8 +257,8 @@ public void testMaterializedViewRequestSent() throws Exception {
   }
 
   class FakeBigtableService extends BigtableGrpc.BigtableImplBase {
-    private final List<SessionRequest> openSessionRequests = new ArrayList<>();
-    private final List<SessionRequest> vrpcRequests = new ArrayList<>();
+    private final List<SessionRequest> openSessionRequests = new CopyOnWriteArrayList<>();
+    private final List<SessionRequest> vrpcRequests = new CopyOnWriteArrayList<>();
 
     @Override
     public void getClientConfiguration(