Remove unreachable error constructors

[Jimbo4350]

Changelog

- description: |
    Remove unreachable error constructors. None of these were produced
    anywhere in the repo — only defined, pretty-printed, and listed in
    golden fixtures.

    Removed:

    - `TransactionValidityError` (entire type, both constructors:
      `TransactionValidityIntervalError`,
      `TransactionValidityCostModelError`).
    - `TxFeeEstimationTransactionTranslationError`, the
      `TxFeeEstimationError` constructor that wrapped the now-removed
      `TransactionValidityError`.
    - `TxBodyErrorByronEraNotSupported`,
      `TxBodyErrorMissingParamMinUTxO`, and
      `TxBodyErrorNonAdaAssetsUnbalanced` from the legacy
      `Cardano.Api.Tx.Internal.Fee.TxBodyErrorAutoBalance`.
    - `TxBodyErrorNonAdaAssetsUnbalanced` from the experimental
      `Cardano.Api.Experimental.Tx.Internal.Fee.TxBodyErrorAutoBalance`.

    The corresponding `test_TransactionValidityError` golden test, the
    three dead `TxBodyErrorAutoBalance` fixture entries, and their
    expected `.txt` files are dropped too. The
    `Ouroboros.Consensus.HardFork.History` import in `Fee.hs` becomes
    unused and is removed.
  type:
   - breaking
  projects:
   - cardano-api

Context

Continuing the pattern of PR #1180 / commit 926606fdd ("Remove unused TxBodyProtocolParamsConversionError constructor"). An audit of the TxBodyError / TxBodyErrorAutoBalance / TxFeeEstimationError / TransactionValidityError types found six error constructors with no production producers — only the data declaration, the prettyError pattern match, and golden fixtures referenced them.

Construction-site search (across cardano-api/src, cardano-api/test, cardano-api/gen) found zero call sites of the form Left X … / X … / first X … / \$ X … for any of the constructors removed here.

Since both TransactionValidityError constructors are dead, the whole GADT is removed, which in turn makes TxFeeEstimationTransactionTranslationError unreachable.

How to trust this PR

cabal build cardano-api -j4
cabal test cardano-api-golden     # 121 / 121 pass

Sanity-check that no producer remains for any removed constructor:

git grep -nE 'TransactionValidity(Interval|CostModel)Error|TxFeeEstimationTransactionTranslationError|TxBodyErrorByronEraNotSupported|TxBodyErrorMissingParamMinUTxO|TxBodyErrorNonAdaAssetsUnbalanced'

After this PR the only matches should be in CHANGELOG.md / release notes.

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
• Changelog fragment added in .changes/ (added in a follow-up commit once PR number is known, matching the project's existing pattern)

[Copilot]

Pull request overview

This PR removes six error constructors from cardano-api's transaction/fee error types that have no producers anywhere in the repo (only definition, pretty-printing, and golden fixtures referenced them). It continues the cleanup pattern from PR #1180. Because both constructors of TransactionValidityError are dead, the entire GADT is dropped, which in turn makes the wrapping TxFeeEstimationTransactionTranslationError unreachable and is also removed.

Changes:

• Drop TransactionValidityError (and its export from Cardano.Api.Tx/Fee), the wrapping TxFeeEstimationTransactionTranslationError, and four dead TxBodyErrorAutoBalance constructors across legacy and experimental modules.
• Remove the corresponding test_TransactionValidityError golden test, three dead TxBodyErrorAutoBalance fixture entries, and their .txt files.
• Drop the now-unused Ouroboros.Consensus.HardFork.History import in Fee.hs and add a breaking Herald changelog fragment.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cardano-api/src/Cardano/Api/Tx/Internal/Fee.hs	Removes TransactionValidityError GADT, TxFeeEstimationTransactionTranslationError, three dead TxBodyErrorAutoBalance constructors, and the now-unused Consensus import.
cardano-api/src/Cardano/Api/Tx.hs	Drops the TransactionValidityError(..) re-export.
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/Fee.hs	Removes TxBodyErrorNonAdaAssetsUnbalanced and its pretty-printer.
cardano-api/test/cardano-api-golden/Test/Golden/ErrorsSpec.hs	Drops test_TransactionValidityError and the three dead TxBodyErrorAutoBalance fixture entries.
cardano-api/test/cardano-api-golden/files/errors/.../TxBodyErrorByronEraNotSupported.txt	Removes the dead golden fixture file.
cardano-api/test/cardano-api-golden/files/errors/.../TxBodyErrorMissingParamMinUTxO.txt	Removes the dead golden fixture file.
cardano-api/test/cardano-api-golden/files/errors/.../TxBodyErrorNonAdaAssetsUnbalanced.txt	Removes the dead golden fixture file.
cardano-api/test/cardano-api-golden/files/errors/.../TransactionValidityCostModelError.txt	Removes the dead golden fixture file.
.changes/20260515_cardano_api_remove_unreachable_error_constructors.yml	Adds a Herald changelog fragment correctly classified as breaking.

[palas]

Solid PR 👍

Only errors and golden tests over those errors were deleted, and it compiles, so I cannot think of any issue with that