Merge pull request #14 from 2023-DEV1-072/negative_test_cases

added test cases for invalid book quantity in priveservice testing

Merge pull request #13 from 2023-DEV1-072/refactoring

moved final values to constants class

Merge pull request #12 from 2023-DEV1-072/refactoring

removed empty lines in class files

Moved the error message to constants class

added negative test cases for controller

added more test cases to controller

refactored code - removed unsed imports,intendation fixes

added enum

Merge pull request #11 from 2023-DEV1-072/exception_handling

fixed typo in message

added exception for invalid book quantity

changed default discount to zero