feat(abstract-utxo): force PSBT format for Bitcoin Testnet hot wallets

Forces PSBT format for Bitcoin Testnet hot wallets and explicitly handles
Zcash separately. This change maintains legacy format for Zcash while
ensuring all other UTXO-based coins on testnet use PSBT format.

Issue: BTC-2732

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -1003,17 +1003,19 @@ export abstract class AbstractUtxoCoin extends BaseCoin {
       return requestedTxFormat as TxFormat;
     }
 
+    if (utxolib.getMainnet(this.network) === utxolib.networks.zcash) {
+      // FIXME(BTC-1322): fix zcash PSBT support
+      return 'legacy';
+    }
+
     const walletFlagMusigKp = wallet.flag('musigKp') === 'true';
     const isHotWallet = wallet.type() === 'hot';
 
     // if not txFormat is already specified figure out if we should default to psbt format
     if (
       wallet.subType() === 'distributedCustody' ||
       // default to testnet for all utxo coins except zcash
-      (isTestnet(this.network) &&
-        // FIXME(BTC-1322): fix zcash PSBT support
-        getMainnet(this.network) !== utxolib.networks.zcash &&
-        isHotWallet) ||
+      (isTestnet(this.network) && isHotWallet) ||
       // if mainnet, only default to psbt for btc hot wallets
       (isMainnet(this.network) && getMainnet(this.network) === utxolib.networks.bitcoin && isHotWallet) ||
       // default to psbt if it has the wallet flag