Clear stale convertedAmount when moving cross-currency expenses from Self DM

When an unreported expense (from Self DM) is moved to a report with a different
currency, the sourceCurrency is undefined. This caused shouldClearConvertedAmount
to bail out early, leaving stale convertedAmount/convertedTaxAmount on the
transaction. Downstream readers (getBillableAndTaxTotal, calculateGroupTotal) then
displayed incorrect Tax and Billable totals while offline.

- Fall back to the transaction's own currency when sourceCurrency is missing
- Clear convertedTaxAmount alongside convertedAmount in optimistic and failure data

Co-authored-by: Aimane Chnaif <aimane-chnaif@users.noreply.github.com>

Author: MelvinBot

src/libs/TransactionUtils/index.ts

@@ -1042,11 +1042,17 @@ function getCurrency(transaction: OnyxInputOrEntry<Transaction>): string {
  * Transactions that match the destination currency can keep their convertedAmount since no conversion is needed.
  */
 function shouldClearConvertedAmount(transaction: OnyxInputOrEntry<Transaction>, sourceCurrency: string | undefined, destinationCurrency: string | undefined): boolean {
-    if (!sourceCurrency || !destinationCurrency || sourceCurrency === destinationCurrency) {
+    if (!destinationCurrency) {
         return false;
     }
 
     const transactionCurrency = getCurrency(transaction);
+    const effectiveSourceCurrency = sourceCurrency ?? transactionCurrency;
+
+    if (effectiveSourceCurrency === destinationCurrency) {
+        return false;
+    }
+
     const transactionMatchesDestination = transactionCurrency === destinationCurrency;
 
     return !transactionMatchesDestination;

src/libs/actions/Transaction.ts

@@ -1089,6 +1089,7 @@ function changeTransactionsReport({
                 originalCurrency: shouldCopyOriginalCurrency ? transaction.originalCurrency : null,
                 ...(shouldClearAmount && {pendingAction: CONST.RED_BRICK_ROAD_PENDING_ACTION.UPDATE}),
                 ...(shouldClearAmount && {convertedAmount: null}),
+                ...(shouldClearAmount && {convertedTaxAmount: null}),
                 ...(oldIOUAction ? {linkedTrackedExpenseReportAction: newIOUAction} : {}),
             },
         });
@@ -1112,6 +1113,7 @@ function changeTransactionsReport({
                 originalCurrency: transaction.originalCurrency,
                 ...(shouldClearAmount && {pendingAction: transaction.pendingAction ?? null}),
                 ...(shouldClearAmount && {convertedAmount: transaction.convertedAmount}),
+                ...(shouldClearAmount && {convertedTaxAmount: transaction.convertedTaxAmount}),
             },
         });