fix(sdk-coin-flr): auto-detect atomic export txs in explainTransaction

When the FLR wallet platform calls explainTransaction for a cross-chain
export without setting crossChainType, the method falls through to the
EVM explain path which cannot parse atomic transaction hex, causing the
recipient P-chain address to be missing from the result.

Avalanche/Flare atomic transactions are identified by their codec prefix
(0x00000000). Add auto-detection: if the tx hex starts with '0000'
(after stripping any 0x prefix), route to explainAtomicTransaction even
when crossChainType is not supplied. This ensures the P-chain recipient
address always appears in outputs[0].address for export transactions,
regardless of whether the caller sets crossChainType explicitly.

Add regression tests in both sdk-coin-flr and sdk-coin-flrp that verify
outputs[0].address is non-empty for C-chain export transactions.

Ticket: CECHO-1445
Session-Id: 9552a57f-ade5-43df-b67b-d19aa62e6e32
Task-Id: 1785a314-e5c6-4b30-b86e-46e56c9206ed

Author: ArunBala-Bitgo

modules/sdk-coin-flr/src/flr.ts

@@ -176,7 +176,10 @@ export class Flr extends AbstractEthLikeNewCoins {
     if (!txHex) {
       throw new Error('missing txHex in explain tx parameters');
     }
-    if (params.crossChainType) {
+    // Avalanche atomic transactions are identified by their codec prefix (0x00000000).
+    // Auto-detect them so callers don't need to pass crossChainType explicitly.
+    const hexWithout0x = txHex.startsWith('0x') ? txHex.slice(2) : txHex;
+    if (params.crossChainType || hexWithout0x.startsWith('0000')) {
       return this.explainAtomicTransaction(txHex);
     }
     if (!params.feeInfo) {

modules/sdk-coin-flr/test/unit/flr.ts

@@ -249,6 +249,19 @@ describe('flr', function () {
       txExplain.changeOutputs.should.be.empty();
     });
 
+    it('should explain an export in C transaction without crossChainType when hex is atomic', async function () {
+      const testData = EXPORT_C_TEST_DATA;
+      // Wallet platform may omit crossChainType; auto-detection via codec prefix must still return
+      // the recipient P-chain address so the UI can display it.
+      const txExplain = await tflrCoin.explainTransaction({
+        txHex: testData.fullsigntxHex,
+      });
+      txExplain.type.should.equal(TransactionType.Export);
+      const sortedPAddresses = testData.pAddresses.slice().sort().join('~');
+      txExplain.outputs[0].address.should.equal(sortedPAddresses);
+      txExplain.outputAmount.should.equal(testData.amount);
+    });
+
     it('should throw error when missing txHex', async function () {
       await tflrCoin
         .explainTransaction({ crossChainType: 'export' } as ExplainTransactionOptions)

modules/sdk-coin-flrp/test/unit/flrp.ts

@@ -282,6 +282,19 @@ describe('Flrp test cases', function () {
       txExplain.changeOutputs.should.be.empty();
     });
 
+    it('should explain a signed export from C-chain transaction and include recipient P-chain address', async () => {
+      const txExplain = await basecoin.explainTransaction({ txHex: EXPORT_IN_C.signedHex });
+
+      txExplain.type.should.equal(TransactionType.Export);
+      txExplain.outputs.should.be.an.Array();
+      txExplain.outputs.length.should.equal(1);
+      // The recipient address must be the P-chain destination — not empty.
+      txExplain.outputs[0].address.should.be.a.String();
+      txExplain.outputs[0].address.should.not.be.empty();
+      txExplain.outputs[0].address.should.startWith('P-');
+      txExplain.changeOutputs.should.be.empty();
+    });
+
     it('should fail when transaction hex is not provided', async () => {
       await basecoin.explainTransaction({}).should.be.rejectedWith('missing transaction hex');
     });