You are tasked with developing a Library Management System in Java. The system should allow librarians to manage the library's collection of books and assist clients in borrowing and returning books. Additionally, clients should be able to search for books by title or author.

Functionality:

$$ When a client borrows a book, update the book's availability status to reflect that it is no longer available.

$$ When a client returns a book, update the book's availability status to reflect that it is available again.

$$ Implement search functionality to allow clients to search for books by title or author.