test(api): add TxOut JSON tests and document case* deprecation

[newhoggy]

Summary

This PR adds comprehensive test coverage for the TxOut JSON instances and documents the deprecation of era case* functions.

Changes

1. Deprecation Notice for Era Case Functions

Adds documentation to Cardano.Api.Era.Internal.Case noting that case* functions are deprecated and will be removed in a future release. New code should use direct pattern matching on era witnesses instead.

2. Comprehensive TxOut JSON Tests

Implements extensive test coverage for the ToJSON and FromJSON instances of TxOut, adapted for the Exp.IsEra constraint on ToJSON.

New test modules:

• Test.Gen.Cardano.Api.TxOut: Specialized generators for TxOut with specific datum types (no datum, datum hash, supplemental, inline) and invalid JSON scenarios
• Test.Cardano.Api.TxOut.Helpers: Test utilities including JSON field assertions, parse failure validators, and datum equality checks
• Test.Cardano.Api.TxOut.Json: Main test module organizing all test suites
• Test.Cardano.Api.TxOut.JsonRoundtrip: Roundtrip property tests for Conway era (CtxTx and CtxUTxO)
• Test.Cardano.Api.TxOut.JsonEdgeCases: Edge case tests for supplemental datum behavior, null field handling, ToJSON output validation
• Test.Cardano.Api.TxOut.JsonErrorCases: Error case tests for conflicting datums, mismatched hashes, partial fields, invalid data

Note: Tests are limited to Conway era because:

• ToJSON now uses Exp.IsEra constraint (Conway/Dijkstra only)
• Dijkstra era is not yet fully supported by shelleyBasedEraConstraints

Test Coverage

• Roundtrip tests: CtxTx and CtxUTxO contexts
• Datum-specific tests: No datum, datum hash, supplemental datum, inline datum
• Error handling: Conflicting datums, mismatched hashes, partial fields, invalid data, missing required fields
• Edge cases: Supplemental datum field behavior, null field handling, ToJSON output validation

Changelog

- description: |
    Add comprehensive JSON roundtrip and error tests for TxOut instances.
    Document deprecation of case* functions in Era.Internal.Case.
  type:
    - test
    - documentation
  projects:
    - cardano-api
    - cardano-api-gen

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

[Jimbo4350]

Blocking until I have time to review. Hopefully be EOD today.

[Jimbo4350]

Where is this being consumed? This function is promoting the propagation of the eon based types we are trying to get rid of e.g AlonzoEraOnwardsAlonzo etc.

[newhoggy]

It's not used. Missed this one when simplifying tests. I have removed it.

[github-actions]

This PR is stale because it has been open 45 days with no activity.