feat: Adds logging when there are no shards during backup and restore

[devanbenz]

We are experiencing a bug where we have null shards for a given ShardGroup in the boltdb metadata. This seems to show up during backup -> restore. Currently we have not been able to reproduce organically. This PR adds logging to backup and restore handlers for when a given ShardGroup metadata is like the following example:

            {
              "ID": 789,
              "StartTime": "2025-12-01T00:00:00Z",
              "EndTime": "2025-12-08T00:00:00Z",
              "DeletedAt": "0001-01-01T00:00:00Z",
              "Shards": null,
              "TruncatedAt": "0001-01-01T00:00:00Z"
            },

Notice there is no DeletedAt and the Shards are null. This is what we want to avoid.

Example for backup:

ts=2025-12-22T23:09:24.489579Z lvl=warn msg="Backup: ShardGroup has not been deleted and has no shards" log_id=0~z4z0lG000 service=bucket-manifest-writer shard-group-id=2 start-time=2025-12-22T17:23:00.000000Z end-time=2025-12-22T17:24:00.000000Z
ts=2025-12-22T23:09:24.489655Z lvl=warn msg="Backup: ShardGroup has not been deleted and has no shards" log_id=0~z4z0lG000 service=bucket-manifest-writer shard-group-id=3 start-time=2025-12-22T18:11:00.000000Z end-time=2025-12-22T18:12:00.000000Z
ts=2025-12-22T23:09:24.489671Z lvl=warn msg="Backup: ShardGroup has not been deleted and has no shards" log_id=0~z4z0lG000 service=bucket-manifest-writer shard-group-id=4 start-time=2025-12-22T18:12:00.000000Z end-time=2025-12-22T18:13:00.000000Z
ts=2025-12-22T23:09:24.489678Z lvl=warn msg="Backup: ShardGroup has not been deleted and has no shards" log_id=0~z4z0lG000 service=bucket-manifest-writer shard-group-id=5 start-time=2025-12-22T22:58:00.000000Z end-time=2025-12-22T22:59:00.000000Z

Example for restoration:

ts=2025-12-22T23:20:24.971567Z lvl=warn msg="Restore: ShardGroup has not been deleted and has no shards" log_id=0~z5Zsi0000 handler=restore shard-group-id=2 start-time=2025-12-22T17:23:00.000000Z end-time=2025-12-22T17:24:00.000000Z
ts=2025-12-22T23:20:24.971590Z lvl=warn msg="Restore: ShardGroup has not been deleted and has no shards" log_id=0~z5Zsi0000 handler=restore shard-group-id=3 start-time=2025-12-22T18:11:00.000000Z end-time=2025-12-22T18:12:00.000000Z
ts=2025-12-22T23:20:24.971594Z lvl=warn msg="Restore: ShardGroup has not been deleted and has no shards" log_id=0~z5Zsi0000 handler=restore shard-group-id=4 start-time=2025-12-22T18:12:00.000000Z end-time=2025-12-22T18:13:00.000000Z
ts=2025-12-22T23:20:24.971602Z lvl=warn msg="Restore: ShardGroup has not been deleted and has no shards" log_id=0~z5Zsi0000 handler=restore shard-group-id=5 start-time=2025-12-22T22:58:00.000000Z end-time=2025-12-22T22:59:00.000000Z