We have contradictory validation checks for aggregation explain commands:
aggregation_request_helper::validate(): we assert that we cannot have explain and writeConcern
in service_entry_point_shard_role we explicitly require all commands to have writeConcern to be explicitly set.

We should update aggregation_request_helper::validate() to permit writeConcern with explain if the command was issued from an internal connection.

We can do this by modifying the assertion condition here to be:

  !hasExplain || !aggregate.getWriteConcern().has_value() || client->isInternalClient() || client->isInDirectClient()

The following may also suffice:

!hasExplain || !aggregate.getWriteConcern().has_value() || hasFromRouterElem

There may be some collision with SERVER-119513 as that will expose the Client* in validate().