Migrate cardano-testnet default era from Conway to Dijkstra

[Jimbo4350]

Summary

cardano-testnet's default test era is currently Conway. Now that Dijkstra support is wired in, we should migrate the default to Dijkstra so general-purpose tests exercise the latest era and we can flush out any code paths that fall through to error on a Dijkstra value.

Current state

The canonical default lives in cardano-testnet/src/Testnet/Start/Types.hs:229:

cardanoNodeEra = AnyShelleyBasedEra ShelleyBasedEraConway

Supporting defaults:

• cardano-testnet/src/Testnet/Defaults.hs:184 — defaultEra :: ShelleyBasedEra ConwayEra
• cardano-testnet/src/Testnet/Defaults.hs:472 — eraToProtocolVersion (already maps Dijkstra → PV 12)

Dijkstra is already in scope — ShelleyBasedEraDijkstra, dijkstraGenesisDefaults, and TestDijkstraHardForkAtEpoch are referenced throughout, so no cardano-api bump is required.

Tests that hardcode Conway only because it's the default (migration candidates)

Generic tests pinned to Conway:

• test/cardano-testnet-test/Cardano/Testnet/Test/FoldEpochState.hs:31
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Query.hs:84
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Transaction.hs:53
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Transaction/RegisterDeregisterStakeAddress.hs:51
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Transaction/BuildEstimate.hs:48
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Plutus/Scripts.hs:56, 206
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Plutus/BuildRaw.hs:61
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/Plutus/CostCalculation.hs:64, 223, 338
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/KesPeriodInfo.hs:68
• test/cardano-testnet-test/Cardano/Testnet/Test/Cli/LeadershipSchedule.hs:68

Governance tests — also migration candidates

All 14 files under test/cardano-testnet-test/Cardano/Testnet/Test/Gov/ (DReps, committee, treasury, proposals, etc.) currently use ConwayEraOnwardsConway / ShelleyBasedEraConway. Governance is ConwayEraOnwards, not Conway-only — these features exist in Dijkstra too, so the tests should run on the latest era to catch era-specific regressions. The ConwayEraOnwardsConway witness in those files is just "use the earliest era that has this feature"; there's no Conway-specific behavior being asserted.

If any individual gov test genuinely depends on pre-Dijkstra behavior that changed, it should be deliberately era-pinned with a comment explaining why — but that's the exception, not the default.

error calls

Only one notable hit, and it's fine as-is — Dijkstra is Conway-onwards, so it won't fire:

• src/Testnet/EpochStateProcessing.hs:41 — error "Governance actions only available in Conway era onwards"

Known gaps

• test/.../Test/Node/Shutdown.hs:132 already TODOs that cardano-cli latest genesis create doesn't yet support Dijkstra — Dijkstra genesis is currently written from dijkstraGenesisDefaults directly. Worth tracking for full parity but not a blocker.
• No genesis.dijkstra.spec.json fixture under cardano-testnet/files/data/, but genesis is generated dynamically so this isn't blocking either.

Proposed approach

1. Flip the default in Testnet/Start/Types.hs:229 to AnyShelleyBasedEra ShelleyBasedEraDijkstra and update Defaults.hs:184 accordingly.
2. Run the test suite and chase any failures.
3. Migrate gov tests' era witnesses from *OnwardsConway to *OnwardsDijkstra (or have them follow the default), unless a specific test legitimately needs Conway pinning.
4. Fix any error paths that surface on Dijkstra.
5. File a follow-up if the cardano-cli genesis-create gap blocks anything.

[palas]

We'll wait until Dijkstra implementation is stabilised before carrying out this issue