This PR includes following changes:

• Wire BackupAgent → DataDistributor coordination for range-partitioned backup partition computation.
• Adds 2 system keys:
  • backupPartitionRequiredKey — todo signal (0/1/2) that BackupAgent writes and DD watches.
  • backupPartitionListKey — partition list (vector) DD writes after computing partitions.
• BackupAgent writes backupPartitionRequired = 1 when the first range-partitioned backup starts, and = 2 when the last finishes, piggybacking on the existing backupStartedKey transaction.
• DataDistributor watches the key; on value 1 it calls calculateBackupPartitionKeyRanges and writes the result to backupPartitionListKey; on value 2 it clears the partition list. In both cases the request key is cleared in the same commit.
• Adds knob BACKUP_NUM_OF_PARTITIONS (default 100) controlling how many partitions calculateBackupPartitionKeyRanges produces.

Coordination between BackupAgent and DataDistributor

• backupPartitionRequired is a small todo key (0/1/2) DD watches via tr.watch(). Multi-backup safe: BackupAgent only writes 1 when adding a UID to an empty backupStartedKey, and only
  writes 2 when removing the last UID; the read-then-write happens in the same txn as the backupStartedKey update.

Testing

100K Completed
20260530-022602-ak_bk_dd_2-43582fe4cfa1abb9 compressed=True data_size=37179731 duration=2238936 ended=100000 fail_fast=10 max_runs=100000 pass=100000 priority=100 remaining=0 runtime=0:26:37 sanity=False started=100000 stopped=20260530-025239 submitted=20260530-022602 timeout=5400 username=ak_bk_dd_2