Refactor friendlyTxBodyImpl with per-feature pair helpers
The 7-way ShelleyBasedEra case duplicated body/mAuxData extraction and a near-identical field list (with Null placeholders) in every branch. Extract one helper per feature (validityRangePair, mintPair, collateralInputsPair, requiredSignersPair, referenceInputsPair, totalCollateralPair, returnCollateralPair) plus alonzoScriptWitnessPairsFor / conwayBodyPairsFor. The top-level body now extracts body/mAuxData once and composes a flat list of pairs. No behaviour change: all golden outputs are unchanged.