[AB2D-6187] increase api test coverage (#1388)

## 🎫 Ticket

https://jira.cms.gov/browse/AB2D-6187

## 🛠 Changes

Adds test coverage for `CustomUserDetailsService`

## ℹ️ Context

This is a part of my journey to achieve 90% test coverage everywhere

---------

Co-authored-by: Anna Smirnova <[email protected]>
Co-authored-by: smirnovaae <[email protected]>

api/src/test/java/gov/cms/ab2d/api/security/CustomUserDetailsServiceTest.java

@@ -0,0 +1,39 @@
+package gov.cms.ab2d.api.security;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.security.core.userdetails.UsernameNotFoundException;
+
+import gov.cms.ab2d.common.model.PdpClient;
+import gov.cms.ab2d.common.repository.PdpClientRepository;
+
+class CustomUserDetailsServiceTest {
+
+  @Test
+  void testLoadUserByUsername1() {
+    PdpClientRepository pdpClientRepository = mock(PdpClientRepository.class);
+    CustomUserDetailsService customUserDetailsService = new CustomUserDetailsService(pdpClientRepository);
+    PdpClient pdpClient = new PdpClient();
+    when(pdpClientRepository.findByClientId("clientId")).thenReturn(pdpClient);
+    assertEquals(
+      customUserDetailsService.loadUserByUsername("clientId"),
+      pdpClient
+    );
+  }
+
+  @Test
+  void testLoadUserByUsername2() {
+    PdpClientRepository pdpClientRepository = mock(PdpClientRepository.class);
+    CustomUserDetailsService customUserDetailsService = new CustomUserDetailsService(pdpClientRepository);
+    when(pdpClientRepository.findByClientId(null)).thenReturn(null);
+    assertThrows(
+      UsernameNotFoundException.class,
+      () -> {customUserDetailsService.loadUserByUsername(null);}
+    );
+  }
+
+}