Create envelope to validate mail

[Bhacaz]

This pull request improves the reliability of the RailsCacheDeliveryMethod by adding envelope validation to ensure that emails have valid sender and recipient addresses before delivery. It also enhances the test suite to cover these validation scenarios. Fix #24

Envelope validation improvements:

• In lib/gravity_mailbox/rails_cache_delivery_method.rb, the deliver! method now creates a Mail::SmtpEnvelope for the email, which validates the presence of sender and recipient addresses before caching the mail.

Test coverage enhancements:

• In spec/gravity_mailbox/rails_cache_delivery_method_spec.rb, the test mail setup now includes explicit from and to addresses to ensure valid envelope data for standard cases.
• Added new test contexts to verify that an error is raised if the mail has no sender, no recipients, or an invalid sender address, ensuring that envelope validation is properly enforced.