test(spanner): unflake multiUseReadOnlyTransactionCanUseInlineBeginForReadAsync

In `SessionImplTest#multiUseReadOnlyTransactionCanUseInlineBeginForReadAsync`,
mock `rpc.read` invoked stream callbacks synchronously before returning `NoOpStreamingCall`.
This caused a background thread to process rows and invoke `startStream` while `this.stream`
was still unassigned, triggering a duplicate request (`expected:<1> but was:<2>`).

Fix this by dispatching callbacks asynchronously with `CompletableFuture.runAsync`,
allowing `rpc.read` to return and assign `this.stream` before row consumption starts.

Author: sakthivelmanii

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionImplTest.java

@@ -66,6 +66,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.TimeZone;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -823,8 +824,12 @@ public void multiUseReadOnlyTransactionCanUseInlineBeginForReadAsync() throws Ex
     Mockito.when(rpc.read(request.capture(), consumer.capture(), anyMap(), any(), eq(false)))
         .then(
             invocation -> {
-              consumer.getValue().onPartialResultSet(resultSet);
-              consumer.getValue().onCompleted();
+              SpannerRpc.ResultStreamConsumer c = invocation.getArgument(1);
+              CompletableFuture.runAsync(
+                  () -> {
+                    c.onPartialResultSet(resultSet);
+                    c.onCompleted();
+                  });
               return new NoOpStreamingCall();
             });