refactor(book-app): add type annotations and input validation

- Add return type annotations to all functions
- Add empty string validation for title and author in handle_add()

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: DanWahlin

samples/book-app-project/book_app.py

@@ -6,7 +6,7 @@
 collection = BookCollection()
 
 
-def show_books(books):
+def show_books(books: list) -> None:
     """Display books in a user-friendly format."""
     if not books:
         print("No books found.")
@@ -21,16 +21,24 @@ def show_books(books):
     print()
 
 
-def handle_list():
+def handle_list() -> None:
     books = collection.list_books()
     show_books(books)
 
 
-def handle_add():
+def handle_add() -> None:
     print("\nAdd a New Book\n")
 
     title = input("Title: ").strip()
+    if not title:
+        print("\nError: Title cannot be empty.\n")
+        return
+
     author = input("Author: ").strip()
+    if not author:
+        print("\nError: Author cannot be empty.\n")
+        return
+
     year_str = input("Year: ").strip()
 
     try:
@@ -41,7 +49,7 @@ def handle_add():
         print(f"\nError: {e}\n")
 
 
-def handle_remove():
+def handle_remove() -> None:
     print("\nRemove a Book\n")
 
     title = input("Enter the title of the book to remove: ").strip()
@@ -50,7 +58,7 @@ def handle_remove():
     print("\nBook removed if it existed.\n")
 
 
-def handle_find():
+def handle_find() -> None:
     print("\nFind Books by Author\n")
 
     author = input("Author name: ").strip()
@@ -59,7 +67,7 @@ def handle_find():
     show_books(books)
 
 
-def show_help():
+def show_help() -> None:
     print("""
 Book Collection Helper
 
@@ -72,7 +80,7 @@ def show_help():
 """)
 
 
-def main():
+def main() -> None:
     if len(sys.argv) < 2:
         show_help()
         return