Migrate to validation >= 1.2

[carbolymer]

Summary

• Migrate cardano-api from the pre-1.2 validation API (toEither, liftError) to direct pattern matching on Failure/Success constructors.
• Add validation >= 1.2 lower bound in cardano-api.cabal.
• Bump hackage index-state to 2026-05-27.

Context

validation 1.2 removed toEither, validate, and liftError.
This was found when bumping hackage.nix in CHaP (IntersectMBO/ouroboros-consensus#2051).

Blocked on a new CHaP release of ouroboros-consensus which also needs the same migration (already done on their main).

Test plan

• CI passes once ouroboros-consensus is released to CHaP with validation 1.2 support

[Copilot]

Pull request overview

Migrates cardano-api off the pre-1.2 validation API by switching internal decoding logic to pattern-match on Validation’s Failure/Success, and updates build inputs (Cabal bounds + pinned package indexes) to support validation >= 1.2.

Changes:

• Update internal witness decoding to accumulate decoder failures via Validation constructors (no toEither/liftError).
• Add a validation >= 1.2 lower bound in cardano-api.cabal.
• Bump cabal.project index-state and flake.lock to newer package sets; add .serena/ to .gitignore.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
flake.lock	Updates pinned hackage.nix input to align with newer package set/index state.
cardano-api/src/Cardano/Api/Tx/Internal/Sign.hs	Replaces removed validation helpers with Failure/Success pattern matching for decoder selection.
cardano-api/cardano-api.cabal	Raises dependency lower bound to validation >= 1.2.
cabal.project	Bumps Hackage index-state to 2026-05-27.
.gitignore	Ignores .serena/ directory.
.changes/20260527_cardano_api_validation_1_2.yml	Adds changelog fragment documenting the migration and index-state bump.