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/BankAccountTransferConsistencyTest.java

@@ -0,0 +1,45 @@
+package banking;
+
+import dev.banking.model.BankAccount;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.*;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class BankAccountTransferConsistencyTest {
+
+    @Test
+    void transferConservation() throws Exception {
+
+        BankAccount a = new BankAccount(1, 100_000);
+        BankAccount b = new BankAccount(2, 100_000);
+
+        ExecutorService executor = Executors.newFixedThreadPool(50);
+        CountDownLatch latch = new CountDownLatch(100);
+
+        for (int i = 0; i < 50; i++) {
+
+            executor.submit(() -> {
+                for (int j = 0; j < 1000; j++) {
+                    a.transfer(b, 1);
+                }
+                latch.countDown();
+            });
+
+            executor.submit(() -> {
+                for (int j = 0; j < 1000; j++) {
+                    b.transfer(a, 1);
+                }
+                latch.countDown();
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        long total = a.getBalance() + b.getBalance();
+
+        assertEquals(200_000, total);
+    }
+}