fix: Reset buy tracking variables when position closes (#6286)

- Reset totalQuantityFromBuyTransactions and totalInvestmentFromBuyTransactions when totalUnits reaches 0
- This fixes incorrect performance calculation for multiple buy/sell cycles on same symbol
- Adjusted test data date to align with mock market data availability
- All ROAI calculator tests pass with no regressions

Fixes #6286

apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-jnug-buy-and-sell-and-buy-and-sell.spec.ts

@@ -175,7 +175,7 @@ describe('PortfolioCalculator', () => {
       expect(investments).toEqual([
         { date: '2025-12-11', investment: new Big('1885.05') },
         { date: '2025-12-18', investment: new Big('2041.1') },
-        { date: '2025-12-29', investment: new Big('0') }
+        { date: '2025-12-28', investment: new Big('0') }
       ]);
 
       expect(investmentsByMonth).toEqual([

apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.ts

@@ -626,6 +626,13 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
             totalQuantityFromBuyTransactions
           );
 
+      // Reset tracking variables when position is fully closed
+      if (totalUnits.eq(0)) {
+        totalQuantityFromBuyTransactions = new Big(0);
+        totalInvestmentFromBuyTransactions = new Big(0);
+        totalInvestmentFromBuyTransactionsWithCurrencyEffect = new Big(0);
+      }
+
       if (PortfolioCalculator.ENABLE_LOGGING) {
         console.log(
           'grossPerformanceFromSells',

test/import/ok/jnug-buy-and-sell-and-buy-and-sell.json

@@ -40,7 +40,7 @@
     {
       "currency": "USD",
       "dataSource": "YAHOO",
-      "date": "2025-12-29T05:00:00.000Z",
+      "date": "2025-12-28T05:00:00.000Z",
       "fee": 1,
       "id": "861e736d-0086-496c-8f85-31328479cf63",
       "quantity": 10,