fix(integration-tests): improve the robustness of the test fixture

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Author: tzolov

spring-ai-integration-tests/src/test/java/org/springframework/ai/integration/tests/tool/domain/BookService.java

@@ -25,15 +25,13 @@
  */
 public class BookService {
 
-	private static final Map<Integer, Book> books = new ConcurrentHashMap<>();
-
-	static {
-		books.put(1, new Book("His Dark Materials", "Philip Pullman"));
-		books.put(2, new Book("Narnia", "C.S. Lewis"));
-		books.put(3, new Book("The Hobbit", "J.R.R. Tolkien"));
-		books.put(4, new Book("The Lord of The Rings", "J.R.R. Tolkien"));
-		books.put(5, new Book("The Silmarillion", "J.R.R. Tolkien"));
-	}
+	private static final ConcurrentHashMap<Integer, Book> books = new ConcurrentHashMap<>(Map
+		.of( // @formatter:off
+		1, new Book("His Dark Materials", "Philip Pullman"),
+		2, new Book("Narnia", "C.S. Lewis"),
+		3, new Book("The Hobbit", "J.R.R. Tolkien"),
+		4, new Book("The Lord of The Rings", "J.R.R. Tolkien"),
+		5, new Book("The Silmarillion", "J.R.R. Tolkien"))); // @formatter:on
 
 	public List<Book> getBooksByAuthor(Author author) {
 		return books.values().stream().filter(book -> author.name().equals(book.author())).toList();