Unify Dijkstra-era placeholder messages under TODO Dijkstra

[Jimbo4350]

Changelog

- description: |
    Unify Dijkstra-era placeholder `error` messages and `TODO` comments
    under the single greppable token `TODO Dijkstra:`, so
    `grep -r 'TODO Dijkstra'` locates every Dijkstra-era placeholder in
    cardano-api in one shot. No behaviour change.
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...
# uncomment at least one main project this PR is associated with
  projects:
   - cardano-api
  # - cardano-api-gen
  # - cardano-rpc
  # - cardano-wasm

Context

Follow-up to #1175. A reviewer there noted that Dijkstra-era placeholders in cardano-node use three different conventions, so no single grep catches them all. The same problem exists in cardano-api: 27 error stubs mentioning Dijkstra, spread across 5+ phrasings (not supported, is not yet supported, not supported yet, currently not supported, is not active yet), plus 4 -- TODO comments in various forms. grep 'TODO.*Dijkstra' missed the error stubs; grep 'Dijkstra era' missed the TODOs; no single grep caught all 31.

This PR unifies 30 of them (see exclusions below) under the literal contiguous token TODO Dijkstra so grep -rn 'TODO Dijkstra' cardano-api/src now returns every Dijkstra-era placeholder a future maintainer needs to audit when Dijkstra-era support lands.

Format applied

• Error stubs: error \"TODO Dijkstra: <function>: <brief reason>\" — preserves existing descriptive context so runtime stack traces remain self-explanatory.
• TODO comments: -- TODO Dijkstra: <brief reason> — colon is after Dijkstra, not between TODO and Dijkstra, otherwise grep 'TODO Dijkstra' misses.

Sites touched (30)

• Case combinators (7): Era/Internal/Case.hs:{55,73,90,107,124,141,158}
• Eon constraints (5): Era/Internal/Eon/{AllegraEraOnwards,MaryEraOnwards,BabbageEraOnwards,ConwayEraOnwards,ShelleyBasedEra}.hs
• Old API (7): Certificate/Internal.hs:{829,846}, Tx/Internal/Body.hs:{2665,3107}, Tx/Internal/Body/Lens.hs:172, Plutus/Internal/Script.hs:{1429,1463}
• Experimental API (6): Experimental/Tx/Internal/Fee.hs:{431,1545}, Experimental/Tx/Internal/AnyWitness.hs:128, Experimental/Tx/Internal/Certificate/Compatible.hs:74, Experimental/Tx/Internal/BodyContent/New.hs:{197,849}
• Serialisation orphan (1): Internal/Orphans/Serialisation.hs:273 (error) + L271 (adjacent TODO)
• TODO comments (4): Internal/Orphans/Serialisation.hs:271, LedgerState.hs:{1168,1514,1515}

Intentional exclusions

• Tx/Internal/Body.hs:2858 — toScriptIndexDijkstra: unexpected DijkstraGuarding at index N is a runtime invariant assertion, not a "Dijkstra era not yet supported" placeholder. DijkstraGuarding is a ledger-side constructor with no corresponding ScriptWitnessIndex case; the error fires only if the ledger ever hands us one. Fixing that would mean adding a new ScriptWitnessIndex variant (a breaking change) — different kind of work from everything else on the list.
• Plutus/Internal/Script.hs:1302 — toShelleyScript: PlutusV4 not implemented yet. is a PlutusV4 stub that blocks on ledger exposing a PlutusV4 constructor. Will be tracked as a dedicated issue rather than prefixed with TODO Dijkstra (it's PlutusV4-gated, not Dijkstra-gated).

How to trust this PR

• Verify the greppability claim:

  grep -rn 'TODO Dijkstra' cardano-api/src | wc -l     # → 30
  grep -rn 'is not yet supported\|currently not supported\|is not active yet\|not supported yet' cardano-api/src  # → no Dijkstra hits
  grep -rn 'DijkstraEra is not supported\|DijkstraEra not supported\|Dijkstra era is not\|Dijkstra era not supported' cardano-api/src  # → no hits
  grep -rn -- '-- TODO: Dijkstra\|TODO: Build dummy dijkstra\|TODO: fix this instance when the Dijkstra' cardano-api/src  # → no hits
  grep -n 'unexpected DijkstraGuarding' cardano-api/src/Cardano/Api/Tx/Internal/Body.hs  # → 1 hit (intentionally preserved)
  

• Only strings/comments changed — no type signatures or logic touched. Diff is 30 insertions / 30 deletions on the .hs files.
• Builds clean: cabal build cardano-api -j4.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• Self-reviewed the diff

[Copilot]

Pull request overview

Standardizes Dijkstra-era placeholder error messages and TODO comments across cardano-api under a single greppable token (TODO Dijkstra:), to make future cleanup/audits easy and reliable without changing behavior.

Changes:

• Replaces varied Dijkstra placeholder error strings with error "TODO Dijkstra: <function>: <reason>".
• Normalizes Dijkstra-related TODO comments to the -- TODO Dijkstra: ... format.
• Adds a Herald changelog fragment marking the change as maintenance.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cardano-api/src/Cardano/Api/Tx/Internal/Body/Lens.hs	Unifies Dijkstra-era placeholder error string for lens accessor.
cardano-api/src/Cardano/Api/Tx/Internal/Body.hs	Unifies Dijkstra-era placeholder error strings in tx body construction/vote extraction.
cardano-api/src/Cardano/Api/Plutus/Internal/Script.hs	Unifies Dijkstra-era placeholder error strings in script conversions.
cardano-api/src/Cardano/Api/LedgerState.hs	Normalizes Dijkstra-related TODO comments around genesis/hard-fork trigger handling.
cardano-api/src/Cardano/Api/Internal/Orphans/Serialisation.hs	Normalizes Dijkstra TODO comment and placeholder ToJSON error for Dijkstra ApplyTxError.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/Fee.hs	Unifies Dijkstra-era placeholder error strings in fee/auto-balance paths.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/Certificate/Compatible.hs	Unifies Dijkstra-era placeholder error string in certificate construction.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/BodyContent/New.hs	Unifies Dijkstra-era placeholder error strings in unsigned tx and vote witness extraction.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/AnyWitness.hs	Unifies Dijkstra-era placeholder error string for Plutus datum extraction.
cardano-api/src/Cardano/Api/Era/Internal/Eon/ShelleyBasedEra.hs	Unifies Dijkstra-era placeholder error string in constraints helper.
cardano-api/src/Cardano/Api/Era/Internal/Eon/MaryEraOnwards.hs	Unifies Dijkstra-era placeholder error string in constraints helper.
cardano-api/src/Cardano/Api/Era/Internal/Eon/ConwayEraOnwards.hs	Unifies Dijkstra-era placeholder error string in constraints helper.
cardano-api/src/Cardano/Api/Era/Internal/Eon/BabbageEraOnwards.hs	Unifies Dijkstra-era placeholder error string in constraints helper.
cardano-api/src/Cardano/Api/Era/Internal/Eon/AllegraEraOnwards.hs	Unifies Dijkstra-era placeholder error string in constraints helper.
cardano-api/src/Cardano/Api/Era/Internal/Case.hs	Unifies Dijkstra-era placeholder error strings in era case combinators.
cardano-api/src/Cardano/Api/Certificate/Internal.hs	Unifies Dijkstra-era placeholder error strings in anchor extraction logic.
.changes/20260420_cardano_api_dijkstra_placeholder_unification.yml	Adds Herald fragment documenting the maintenance-only string/TODO unification.