Fix ActiveUserCountThresholdWorker to actually send email

What does this MR do and why?

Fix ActiveUserCountThresholdWorker to actually send email

The worker was building the mailer, but not calling deliver_later, so the email was never actually sent. This adds the missing deliver_later call to ensure the license threshold notification is delivered.

Also updates the spec to use have_enqueued_mail matcher instead of stubbing mailer methods, providing better test coverage.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start your local GitLab instance with an Ultimate license and check if the worker really triggers an email by executing the following code:

license = License.current
license.define_singleton_method(:active_user_count_threshold_reached?) { true }
license.define_singleton_method(:remaining_user_count) { 1 }

ActionMailer::Base.deliveries.clear
ActionMailer::Base.delivery_method = :test
ActiveJob::Base.queue_adapter = :inline

ActiveUserCountThresholdWorker.new.perform

mail = ActionMailer::Base.deliveries.last
puts mail.subject  # "Your subscription is nearing its user limit"
puts mail.bcc      # Admin emails

MR acceptance checklist

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