feat: Check if shards len is less than or equal to 1

[devanbenz]

Might be related to bug we are seeing where we end up with ShardGroups that have null shards without a DeleteAt time like the following, but very unlikely. 0 as an invariant is not possible afaik:

            {
              "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"
            },

Related logging PR: #27035