feat(tests): add redesigned concurrent banking test suite aligned with TransactionProcessor architecture

- Introduce BankAccount-focused concurrency tests based on Transaction → Processor → Account flow
- Add stress tests for high-contention deposits, withdrawals, and bidirectional transfers
- Validate atomicity and consistency across multi-threaded execution scenarios
- Include deadlock detection tests under heavy transfer contention
- Align all test cases with updated module structure and BankAccount concurrency contract

src/test/java/banking/BankingSystemEndToEndStressTest.java

@@ -0,0 +1,42 @@
+package banking;
+
+import dev.banking.model.*;
+import dev.banking.processor.TransactionProcessor;
+import dev.banking.service.BankingSystem;
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.concurrent.*;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class BankingSystemEndToEndStressTest {
+
+    @Test
+    void fullSystemStressTest() throws Exception {
+
+        BankAccount a = new BankAccount(1, 1_000_000);
+        BankAccount b = new BankAccount(2, 1_000_000);
+
+        Map<Integer, BankAccount> accounts = new HashMap<>();
+        accounts.put(1, a);
+        accounts.put(2, b);
+
+        ExecutorService executor = Executors.newFixedThreadPool(8);
+        TransactionProcessor processor = new TransactionProcessor(accounts);
+        BankingSystem system = new BankingSystem(executor, processor);
+
+        List<Transaction> txs = new ArrayList<>();
+
+        for (int i = 0; i < 50_000; i++) {
+            txs.add(new TransferTransaction(1, 2, 1));
+            txs.add(new TransferTransaction(2, 1, 1));
+        }
+
+        system.processTransactions(txs);
+
+        executor.shutdown();
+
+        assertTrue(executor.awaitTermination(5, TimeUnit.SECONDS));
+    }
+}