[sim] "Not enough datasets" 503 on create disk

I don't know when this was introduced bcause I haven't tried to create a disk like this in a while. When I run sim sled agent locally (on Mac — possibly relevant) with `omicron-dev run-things generally work except for creating disks. When I create a disk with the CLI like this:

oxide disk create --project prod-online --json-body /dev/stdin <<EOF
    {
      "name": "nginx",
      "description": "The nginx disk.",
      "size": 1073741824,
      "disk_source": {
        "type": "blank",
        "block_size": 512
      }
    }
EOF

I get a 503 with this internal message in the logs:

saga error at node "datasets_and_regions": Not enough datasets to allocate disks

That error comes from here:

https://github.com/oxidecomputer/omicron/blob/857a9b95a83e7c833822fc29e3b89c4d1be41386/nexus/db-queries/src/db/queries/region_allocation.rs#L332-L373

I am not shocked to find there are no regions, zpools, or datasets in the DB:

~/oxide/omicron $ cockroach sql --insecure --url "postgresql://root@127.0.0.1:64043/omicron?sslmode=disable"                                    main
#
# Welcome to the CockroachDB SQL shell.
# All statements must be terminated by a semicolon.
# To exit, type: \q.
#
# Server version: CockroachDB CCL v22.1.9 (x86_64-apple-darwin19, built 2022/10/17 18:39:01, go1.17.11) (same version as client)
# Cluster ID: d494037f-10a5-425a-ae38-bed344cd7148
#
# Enter \? for a brief introduction.
#
root@127.0.0.1:64043/omicron> select * from zpool;
  id | time_created | time_modified | time_deleted | rcgen | sled_id | physical_disk_id | total_size
-----+--------------+---------------+--------------+-------+---------+------------------+-------------
(0 rows)


Time: 2ms total (execution 1ms / network 0ms)

root@127.0.0.1:64043/omicron> select * from region;
  id | time_created | time_modified | dataset_id | volume_id | block_size | blocks_per_extent | extent_count
-----+--------------+---------------+------------+-----------+------------+-------------------+---------------
(0 rows)


Time: 2ms total (execution 1ms / network 0ms)

root@127.0.0.1:64043/omicron> select * from dataset;
  id | time_created | time_modified | time_deleted | rcgen | pool_id | ip | port | kind | size_used
-----+--------------+---------------+--------------+-------+---------+----+------+------+------------
(0 rows)


Time: 4ms total (execution 3ms / network 0ms)

Start with nexus/db-queries/src/db/queries/region_allocation.rs around the linked allocation logic, then reproduce the issue with the shown oxide disk create command under the sim sled agent and omicron-dev setup. Check the local database state for the missing regions, zpools, and datasets; done means disk creation no longer returns the 503.