Remove redundant ReturnNull check + update Javadoc string

Author: codesungrape

src/main/java/com/codesungrape/hmcts/bookapi/service/BookService.java

@@ -23,7 +23,6 @@ public class BookService {
      * @return The saved Book entity
      * @throws NullPointerException     if request is null
      * @throws IllegalArgumentException if title is null or blank
-     * @throws IllegalStateException    if the repository fails to save the book
      */
     public Book createBook(BookRequest request) {
         // Validation check for business rules (e.g., uniqueness, if required)

src/test/java/com/codesungrape/hmcts/bookapi/BookServiceTest.java

@@ -17,16 +17,23 @@
 import java.util.UUID;
 import java.util.stream.Stream;
 
-import static org.junit.jupiter.api.Assertions.*;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.*;
-
-/**
- * @ExtendWith(MockitoExtension.class): tells JUnit 5 to use Mockito's extension and automatically
- * initializes all @Mock and @InjectMocks fields when running this test class. @Mock: Creates a fake
- * version (mock) of the dependency. @InjectMocks: creates an instance of the real class under
- * test. @BeforeEach: Runs before each test method in the class. @Test: Marks the method as a test
- * case that JUnit should execute.
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.when;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.times;
+
+
+/**.
+ * Explains how this test class uses Mockito:
+ * - JUnit is extended using MockitoExtension
+ * - @Mock creates fake dependencies
+ * - @InjectMocks creates the real service with mocks injected
+ * - @BeforeEach runs before every test
+ * - @Test marks a test method
  */
 
 // Annotation tells JUnit to use Mockito
@@ -203,20 +210,6 @@ void testCreateBook_RepositoryFailure_ThrowsException() {
 
     // ----- EDGE cases ---------
 
-    @Test
-    void testCreateBook_RepositoryReturnsNull_HandlesGracefully() {
-        // Arrange
-        when(testBookRepository.save(any(Book.class))).thenReturn(null);
-
-        // Act & assert
-        assertThrows(
-            IllegalStateException.class,
-            () -> {
-                testBookService.createBook(validBookRequest);
-            }
-        );
-    }
-
     @ParameterizedTest(name = "{0}") // Display the test name
     @MethodSource("provideLongFieldTestCases")
     void testCreateBook_VeryLongFields_Success(