[reconfigurator] Fix DatasetsEditor internal zpool,kind -> ID map

Blueprint zones don't contain explicit dataset IDs (yet; see #7214), so
`DatasetsEditor` maintained an internal cache mapping `(zpool, kind) ->
dataset ID`. However, that mapping is only unique for _in service_
datasets, and `DatasetsEditor` was erroneously trying to build it for
_all_ datasets (both in-service and expunged).

This PR adds a few property tests and fixes for the maintenance of this
cache, and should ensure that we only try to maintain the "at most one
dataset of a given kind on a given zpool" map for in-service datasets.

common/src/api/internal/shared.rs

@@ -872,6 +872,7 @@ pub struct ExternalIpGatewayMap {
 
 /// Describes the purpose of the dataset.
 #[derive(Debug, Clone, PartialEq, Eq, Ord, PartialOrd, Hash, EnumCount)]
+#[cfg_attr(feature = "testing", derive(test_strategy::Arbitrary))]
 pub enum DatasetKind {
     // Durable datasets for zones
     Cockroach,

nexus/reconfigurator/planning/Cargo.toml

@@ -39,6 +39,7 @@ omicron-workspace-hack.workspace = true
 [dev-dependencies]
 expectorate.workspace = true
 maplit.workspace = true
+omicron-common = { workspace = true, features = ["testing"] }
 omicron-test-utils.workspace = true
 proptest.workspace = true
 test-strategy.workspace = true