[RED] Add unit tests for OTPService to validate OTP generation and verification

src/test/java/com/safetypin/authentication/service/OTPServiceTest.java

+package com.safetypin.authentication.service;
+
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Constructor;
+import java.time.LocalDateTime;
+import java.util.concurrent.ConcurrentHashMap;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class OTPServiceTest {
+
+    @Test
+    void testGenerateOTP() {
+        OTPService otpService = new OTPService();
+        String email = "user@example.com";
+        String otp = otpService.generateOTP(email);
+        assertNotNull(otp, "OTP should not be null");
+        assertEquals(6, otp.length(), "OTP should be 6 characters long");
+        assertTrue(otp.matches("\\d{6}"), "OTP should consist of 6 digits");
+    }
+
+    @Test
+    void testVerifyOTPSuccess() {
+        OTPService otpService = new OTPService();
+        String email = "user@example.com";
+        String otp = otpService.generateOTP(email);
+        // Immediately verify the generated OTP; it should succeed.
+        boolean result = otpService.verifyOTP(email, otp);
+        assertTrue(result, "The OTP should verify successfully");
+    }
+
+    @Test
+    void testVerifyOTPWrongOtp() {
+        OTPService otpService = new OTPService();
+        String email = "user@example.com";
+        otpService.generateOTP(email);
+        // Try verifying with an incorrect OTP.
+        boolean result = otpService.verifyOTP(email, "000000");
+        assertFalse(result, "Verification should fail for an incorrect OTP");
+    }
+
+    @Test
+    void testVerifyOTPExpired() throws Exception {
+        OTPService otpService = new OTPService();
+        String email = "user@example.com";
+        String otp = otpService.generateOTP(email);
+
+        // Access the private otpStorage field via reflection.
+        java.lang.reflect.Field otpStorageField = OTPService.class.getDeclaredField("otpStorage");
+        otpStorageField.setAccessible(true);
+        @SuppressWarnings("unchecked")
+        ConcurrentHashMap<String, Object> otpStorage = (ConcurrentHashMap<String, Object>) otpStorageField.get(otpService);
+
+        // Retrieve the current OTPDetails instance.
+        Object oldOtpDetails = otpStorage.get(email);
+        assertNotNull(oldOtpDetails, "OTPDetails instance should exist");
+
+        // Use reflection to get the private constructor of OTPDetails.
+        Class<?> otpDetailsClass = oldOtpDetails.getClass();
+        Constructor<?> constructor = otpDetailsClass.getDeclaredConstructor(String.class, LocalDateTime.class);
+        constructor.setAccessible(true);
+        // Create a new OTPDetails instance with an expired time (3 minutes ago).
+        Object expiredOtpDetails = constructor.newInstance(otp, LocalDateTime.now().minusMinutes(3));
+        // Replace the old OTPDetails with the expired one.
+        otpStorage.put(email, expiredOtpDetails);
+
+        // Now verification should fail because the OTP is expired.
+        boolean result = otpService.verifyOTP(email, otp);
+        assertFalse(result, "The OTP should be expired and verification should fail");
+    }
+
+    @Test
+    void testVerifyOTPWhenNotGenerated() {
+        OTPService otpService = new OTPService();
+        // No OTP was generated for this email, so verification should return false.
+        boolean result = otpService.verifyOTP("nonexistent@example.com", "123456");
+        assertFalse(result, "Verification should fail when no OTP is generated for the given email");
+    }
+}