Name the unused Mockito answer parameter for Java 21 compatibility

ReplicationQueueTest used an unnamed variable (`_`) as the thenAnswer
lambda parameter. Unnamed variables are a standard feature only in
Java 22+; under a Java 21 language level they are a disabled preview
feature and fail to compile. Give the parameter a name so the test
builds on both Java 21 and Java 25.

Change-Id: Ib121547711a022a1ec1304289b905cb6c5f54428