Implement ToJSON (Dijkstra.ApplyTxError DijkstraEra) — currently stubbed with error

[Jimbo4350]

Summary

In cardano-api/src/Cardano/Api/Internal/Orphans/Serialisation.hs:272, the ToJSON instance for Dijkstra.ApplyTxError Consensus.DijkstraEra is stubbed out with a runtime error:

```haskell
instance ToJSON (Dijkstra.ApplyTxError Consensus.DijkstraEra) where
toJSON = error "Dijkstra era is not active yet"
```

Task

Once the Dijkstra era is active (or as part of activation work), replace this stub with a proper ToJSON implementation — following the pattern used for other eras (e.g. Conway, Babbage).

Notes

• Leaving a error in a typeclass instance is a runtime footgun; this should be tracked and resolved before Dijkstra goes live.
• Related to the broader Dijkstra era integration work.