Add Validations and Error Handling

[mclancy96]

This pull request introduces several improvements for Ruby 3.3+ compatibility, adds model validations, and enhances error handling in controllers. The most important changes are grouped below by theme.

Ruby 3.3+ Compatibility and Dependency Updates:

• Added gems (bigdecimal, fiddle, logger, mutex_m, ostruct, nio4r) to the Gemfile to prepare for Ruby 3.3/3.4/3.5 stdlib removals and updated sqlite3 to a compatible version. Also updated platform array syntax for consistency. [1] [2]
• Updated config/boot.rb to explicitly require logger for ActiveSupport compatibility with Ruby 3.3+.
• Modified config/database.yml to remove YAML aliases for compatibility with Ruby 3.3+ and Psych changes.

Model Validations:

• Added validations in Author for presence and minimum length of name, and uniqueness of email.
• Added validations in Post for presence of title, inclusion of category in allowed values, and minimum length for content.

Controller Error Handling Improvements:

• Updated AuthorsController and PostsController to use create!/update! and rescue ActiveRecord::RecordInvalid, returning structured error responses for validation failures. [1] [2] [3]