Prevent errors for conflicting pending escalations

What does this MR do and why?

• Resolves uncaught Rule has already been taken error when an escalatable is queued for escalation multiple times.
• Makes the {Alert/Issue}CreateWorkers actually idempotent.

How to set up and validate locally

1. Open a project for which you have maintainer permissions (ids needed for command below)
2. Create an escalation policy (project > Monitor > Escalation Policies) with multiple rules:
   1. One rule should fire after 0 minutes
   2. Another rule should fire after 5+ minutes
      
3. Create an incident & trigger escalations

   user = User.first # your user
   project = Project.find(21) # your project
   incident = IncidentManagement::Incidents::CreateService.new(project, user, title: 'incident title', description: 'description').execute[:issue]
   IncidentManagement::PendingEscalations::CreateService.new(incident.escalation_status).execute
   # => Should return an array of strings with as many elements as escalation rules you configured; escalations were created!

4. Before the 2nd escalation rule fires, queue escalations again

   IncidentManagement::PendingEscalations::CreateService.new(incident.escalation_status).execute
   # => Should return nil; escalations should not be created

BEFORE

If you ran the above code on master, the last step would result in an error:

IncidentManagement::PendingEscalations::CreateService.new(incident.escalation_status).execute
# => ActiveRecord::RecordInvalid: Validation failed: Rule has already been taken

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #372194 (closed)