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/BankAccountTransferDeadlockTest.java

@@ -0,0 +1,35 @@
+package banking;
+
+import dev.banking.model.BankAccount;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.*;
+
+import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
+
+import java.time.Duration;
+
+public class BankAccountTransferDeadlockTest {
+
+    @Test
+    void deadlockFreeTransfers() {
+
+        assertTimeoutPreemptively(Duration.ofSeconds(5), () -> {
+
+            BankAccount a = new BankAccount(1, 1_000_000);
+            BankAccount b = new BankAccount(2, 1_000_000);
+
+            ExecutorService executor = Executors.newFixedThreadPool(50);
+
+            for (int i = 0; i < 50_000; i++) {
+
+                executor.submit(() -> a.transfer(b, 1));
+                executor.submit(() -> b.transfer(a, 1));
+            }
+
+            executor.shutdown();
+
+            executor.awaitTermination(5, TimeUnit.SECONDS);
+        });
+    }
+}