Backport: Extract pipeline refresh logic into dedicated service and worker

What does this MR do and why?

This MR backports the extraction of pipeline refresh logic from the main MergeRequests::RefreshService into dedicated service and worker classes. This is part of an effort to split the monolithic refresh service into smaller, more focused components.

What it does:

• Creates MergeRequests::Refresh::PipelineService to handle pipeline refresh logic
• Creates MergeRequests::Refresh::PipelineWorker to execute pipeline refresh asynchronously
• Introduces the split_refresh_worker_pipeline feature flag (default: enabled) to control the new behavior
• When the feature flag is enabled, pipeline refresh is delegated to the new worker instead of being executed inline
• Also includes backport of approval refresh logic extraction with MergeRequests::Refresh::ApprovalService and MergeRequests::Refresh::ApprovalWorker
• Updates the split_refresh_worker_web_hooks feature flag from wip to beta status with default enabled

Why: This refactoring improves the architecture by:

• Reducing the complexity of the main RefreshService
• Allowing pipeline refresh to be executed asynchronously, improving performance
• Making the codebase more maintainable and testable
• Following the pattern established for other refresh operations (webhooks, approvals)

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.