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/TransactionProcessorMixedTransactionIntegrationTest.java

@@ -0,0 +1,47 @@
+package banking;
+
+import dev.banking.model.*;
+import dev.banking.processor.TransactionProcessor;
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.concurrent.*;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+public class TransactionProcessorMixedTransactionIntegrationTest {
+
+    @Test
+    void mixedTransactionsShouldExecuteCorrectly() {
+
+        BankAccount a = new BankAccount(1, 1000);
+        BankAccount b = new BankAccount(2, 2000);
+        BankAccount c = new BankAccount(3, 1500);
+
+        Map<Integer, BankAccount> accounts = new HashMap<>();
+        accounts.put(1, a);
+        accounts.put(2, b);
+        accounts.put(3, c);
+
+        TransactionProcessor processor = new TransactionProcessor(accounts);
+
+        ExecutorService executor = Executors.newFixedThreadPool(8);
+
+        List<Transaction> txs = List.of(
+                new DepositTransaction(1, 100),
+                new WithdrawTransaction(2, 50),
+                new TransferTransaction(1, 2, 30),
+                new TransferTransaction(2, 3, 70),
+                new DepositTransaction(3, 200)
+        );
+
+        assertDoesNotThrow(() -> {
+            for (Transaction tx : txs) {
+                executor.submit(() -> processor.process(tx));
+            }
+
+            executor.shutdown();
+            executor.awaitTermination(5, TimeUnit.SECONDS);
+        });
+    }
+}