Adding ability to access the GOV.UK Notify response

[Hassanmir92]

Why

Mail::Notify::DeliveryMethod defines attr_accessor :response but never assigns the response returned by Notifications::Client#send_email.

As a result:

• delivery_method.response is always nil
• Consumers cannot access the GOV.UK Notify notification ID after sending

Since Notify already returns a response object synchronously, this information is available but not set.

What

This PR updates the deliver! method to assign the return value of client.send_email to self.response.

This:

• Preserves existing return behaviour.
• Makes the Notify response accessible via message.delivery_method.response

This enables auditing, correlation, and delivery status tracking without custom overrides.