Handle Praefect replication lag in CreatePipelineWorker

What does this MR do and why?

This MR relates to #504460 and handles a transient failure in CreatePipelineWorker where pipeline creation fails with "Reference not found" because Praefect has not yet replicated a newly pushed ref to all nodes.

Worker changes (app/workers/create_pipeline_worker.rb)

• Introduces PraefectReplicationRetryError, a RetryError subclass, so Sidekiq retries the job without reporting the error to Sentry
• Raises this error when all three conditions are met:
  1. The pipeline error is "Reference not found"
  2. The push is for a new branch/tag (blank before SHA)
  3. The FF ci_create_pipeline_worker_retry_on_reference_not_found is enabled

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

References

• Rollout issue #600554
• Work item #504460