fix: resolve test failures in PR #13

[vikram-blaxel]

Automated fix for failing tests in #13.

Changes made:
The tests were failing because the books table had a NOT NULL constraint on the publisher column, but the tests were not providing a publisher value when creating new book instances. To resolve this, I updated the Book SQLAlchemy model to make publisher nullable and updated the BookIn and BookOut Pydantic models to include an optional publisher field. I also updated the create_book and update_book functions in repositories.py to correctly handle the publisher field and updated db.sql to match the new schema.