Add worker-commands variant to TaskQueuePartition proto

Add WorkerCommandsPartitionId empty message and use it as a new
variant in the TaskQueuePartition oneof, so internal RPCs can
represent worker-commands partitions. Handle the new variant in
PartitionFromPartitionProto. Also update the TaskQueuePartition
comment to describe it as the internal equivalent of the public
TaskQueue proto.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82