feat(abstract-utxo): enable legacy tx format for mainnet

Changes the supportedTxFormats property from readonly to mutable
and sets legacy format support based on network type. Legacy
format is now enabled for mainnet networks while remaining
disabled for testnet.

Issue: BTC-0

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

Author: OttoAllmendinger

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -247,8 +247,9 @@ export interface ExplainTransactionOptions<TNumber extends number | bigint = num
   decodeWith?: SdkBackend;
 }
 
-export interface DecoratedExplainTransactionOptions<TNumber extends number | bigint = number>
-  extends ExplainTransactionOptions<TNumber> {
+export interface DecoratedExplainTransactionOptions<
+  TNumber extends number | bigint = number,
+> extends ExplainTransactionOptions<TNumber> {
   changeInfo?: { address: string; chain: number; index: number }[];
 }
 
@@ -389,8 +390,9 @@ export interface VerifyUserPublicKeyOptions {
   txParams: TransactionParams;
 }
 
-export interface VerifyTransactionOptions<TNumber extends number | bigint = number>
-  extends BaseVerifyTransactionOptions {
+export interface VerifyTransactionOptions<
+  TNumber extends number | bigint = number,
+> extends BaseVerifyTransactionOptions {
   txPrebuild: TransactionPrebuild<TNumber>;
   txParams: TransactionParams;
   wallet: UtxoWallet;
@@ -421,9 +423,9 @@ export abstract class AbstractUtxoCoin
 
   public readonly amountType: 'number' | 'bigint';
 
-  protected readonly supportedTxFormats: { readonly psbt: boolean; readonly legacy: boolean } = {
+  protected supportedTxFormats: { psbt: boolean; legacy: boolean } = {
     psbt: true,
-    legacy: false,
+    legacy: this.isMainnet(),
   };
 
   protected constructor(bitgo: BitGoBase, amountType: 'number' | 'bigint' = 'number') {