Add unit tests with Brain Monkey

[shazzad]

Summary

Add unit tests using Brain Monkey and Mockery to verify core logic without requiring a running WordPress environment.

Changes

• Add brain/monkey and mockery/mockery as dev dependencies
• Add phpunit.xml.dist configuration and test bootstrap
• Add unit tests covering:
  • Integration license methods — get_license_option(), get_license_code(), has_license_code(), is_license_active()
  • Integration transient management — clear_updates_transient() (move, create, initialize)
  • Integration API requests — api_request() success/error paths, license params, fixture parsing
  • Updater pre_set_transient — version comparison, response/no_update management, error handling
• Add .gitattributes to exclude tests from distribution packages
• Update README installation instructions to use composer require

Verification

composer test