Add FromJSON instance for new experimental TxOut

[Jimbo4350]

Context

Implements FromJSON instances for the new TxOut era type (which wraps L.TxOut era) in the experimental API, matching the JSON format produced by the ToJSON instances added in #1176.

Closes #926

Changelog

- description: |
    Add FromJSON instance for new experimental TxOut type with per-era
    concrete instances, inline datum parsing from inlineDatumRaw with
    hash validation, and reference script support via scriptInAnyLangToLedgerScript
  type:
   - feature
  projects:
   - cardano-api

How to trust this PR

• Concrete FromJSON instances for each era (Shelley through Conway), mirroring the ToJSON structure from Add ToJSON instance for new experimental TxOut #1176
• Pre-Alonzo eras parse address and value only via shared txOutBaseParseJson helper
• Alonzo adds datum hash parsing via dataHashTxOutL
• Babbage+ adds inline datum parsing from inlineDatumRaw (hex-encoded original CBOR bytes) with hash validation against inlineDatumhash, and reference script parsing via scriptInAnyLangToLedgerScript
• Supplemental datums deliberately unsupported — the ledger TxOut does not carry them
• Helper functions: addrFromJson (reverse of addrToJson), scriptInAnyLangToLedgerScript (reverse of ledgerScriptToScriptInAnyLang)
• Property test verifies JSON round-trip (fromJSON . toJSON = id) across all Shelley-based eras

Checklist

• Commit sequence is logical and each commit has a good commit message
• New tests are added if needed and old tests still pass
• Code is formatted with fourmolu
• Self-reviewed the diff

[github-actions]

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

[Copilot]

Pull request overview

This PR adds FromJSON support for the experimental TxOut era wrapper (around L.TxOut era), intended to be the inverse of the ToJSON format introduced in #1176, and adds a property test to verify JSON round-tripping for supported Shelley-based eras.

Changes:

• Implement FromJSON instances for experimental TxOut across Shelley→Conway, including Babbage+ inline datum parsing (from inlineDatumRaw) with hash validation and reference script support.
• Refactor datum/reference-script JSON field emission to a shared helper (datumAndRefScriptFields).
• Add a Hedgehog property test for encode/eitherDecode round-tripping, plus a Herald changelog fragment.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cardano-api/test/cardano-api-test/Test/Cardano/Api/Json.hs	Adds a JSON round-trip property for the new experimental TxOut (with Dijkstra currently skipped).
cardano-api/src/Cardano/Api/Experimental/Tx/Internal/BodyContent/New.hs	Adds FromJSON parsing for experimental TxOut (base fields, datum hash, inline datum raw+hash validation, reference scripts).
.changes/20260420_cardano_api_fromjson_experimental_txout.yml	Adds the required changelog fragment documenting the new feature.

[palas]

Looks good. I just made a couple of small suggestions.

inlineDatumhash is really weird, both the capitalisation and it being omitted on Babbage when it inline datum is not present (instad of just being null). But it is outside of the scope of the PR

[palas]

I think this usage of cast could use a comment explaining how it works

[palas]

Suggested change

	Nothing -> case cast (L.coin mv) of
	Just v -> pure v
	Nothing -> fail "txOutBaseParseJson: value is unsupported for this era"
	Nothing -> do
	let L.MaryValue _ ma = mv
	unless (ma == mempty) $
	fail "txOutBaseParseJson: ada-only era output cannot carry a multi-asset value"
	case cast (L.coin mv) of
	Just v -> pure v
	Nothing -> fail "txOutBaseParseJson: value is unsupported for this era"

I think it would be for the best to check there are no non-ada assets in the case of pre-mary eras

[palas]

Suggested change

	case mDatumHash of
	Just dh -> pure $ L.DatumHash dh
	Nothing -> pure L.NoDatum
	maybe L.NoDatum L.DatumHash mDatumHash

Maybe a good place to use the maybe function.