Adds custom email incorrect_forwarding_target error

Feature context

Custom email for Service Desk allows customers to use their own email address for support communication. They forward all emails to the project-specific service desk email address (generated from incoming_email) and provide their SMTP credential for the custom email address. We send all outgoing Service Desk emails using the provided credentials.

Before we enable this, we send a verification email to check the provided credentials work and that the service provider supports all the features we need and email forwarding is set up correctly.

What does this MR do and why?

Solves Custom email verification should fail when usin... (#426128 - closed)

Feature issue Configurable e-mail address for service desk (#329990 - closed)

Adds incorrect_forwarding_target to Service Desk custom email verification errors.

You can add Service Desk issues using an email generated from incoming_email and the additional alias email generated from service_desk_email. Only the one generated from incoming_email supports all features we need for the custom email feature. We received feedback that customers used the other email address, so I wanted to add a check and an error for this case.

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Custom email configuration

Before	After
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Verification result email

Before	After
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How to set up and validate locally

1. If you haven't set up incoming_email and service_desk_email for email ingestion, please add this to your gitlab.yml file in the development: section. Please restart GDK with gdk restart:

   incoming_email:
     enabled: true
     address: "incoming+%{key}@example.com"
   service_desk_email:
     enabled: true
     address: "contact+%{key}@example.com"

   This will allow you to see the Service Desk section in the settings and generate project-specific email addresses.
2. Select a project and user

   project = Project.find(7)
   current_user = User.first

3. Create Service Desk custom email records and mark verification as started

   custom_email = 'support@example.com'
   
   setting = ServiceDeskSetting.find_or_create_by!(project_id: project.id)
   setting.update!(custom_email: custom_email)
   
   credential = ServiceDesk::CustomEmailCredential.create!(
     project_id: project.id,
     smtp_address: 'smtp.gmail.com', # we need a valid smtp address
     smtp_port: 587,
     smtp_username: custom_email,
     smtp_password: 'supersecret'
   )
   
   verification = ServiceDesk::CustomEmailVerification.new(
     project_id: project.id
   )
   verification.mark_as_started!(current_user)

4. Now build the verification email that has the wrong forwarding address in the Delivered-To header. This is the additional Service Desk alias email address. We need the Service Desk address created from the incoming_email address. Then ingest the email and reset the verification object. In the print you should find a line like this error: "incorrect_forwarding_target".

   service_desk_alias_address = project.service_desk_alias_address
   verification_token = verification.token
   
   email_raw = <<~EMAIL
     Delivered-To: #{service_desk_alias_address}
     To: support+verify@example.com
     From: Flight Support <support@example.com>
     Subject: Verify custom email address support@example.com for Flight
     Auto-Submitted: no
   
   
     This email is auto-generated. It verifies the ownership of the entered Service Desk custom email address and
     correct functionality of email forwarding.
   
     Verification token: #{verification_token}
     --
   
     You're receiving this email because of your account on 127.0.0.1.
   EMAIL
   
   ServiceDeskEmailReceiverWorker.new.perform(email_raw)
   
   verification.reset

5. Open http://127.0.0.1:3000/flightjs/Flight/edit and expand the Service Desk section and find the Configure a custom email card. You should see the :incorrect_forwarding_target error with a short description that contains the correct forwarding address.
6. Open http://127.0.0.1:3000/rails/mailers/notify/service_desk_verification_result_email_for_incorrect_forwarding_target_error.html?locale=en to view the email preview for the Service Desk custom email verification result email. This should show the incorrect_forwarding_target error. When running through the verification process project owners and the person who triggered the verification will receive an email once a verification result is available.
7. (Optional) Clean up custom email and disable flag

   ::ServiceDesk::CustomEmails::DestroyService.new(
     project: project,
     current_user: current_user
   ).execute

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