Split Pipeline logic from MergeRequests::RefreshService

What does this MR do and why?

This MR splits pipeline refresh logic from MergeRequests::RefreshService into a dedicated service and background worker as part of our ongoing effort to decompose the monolithic refresh service.

Changes:

• Creates MergeRequests::Refresh::PipelineService to handle pipeline refresh logic
• Adds MergeRequests::Refresh::PipelineWorker to process pipeline refreshes asynchronously
• Introduces feature flag split_refresh_worker_pipeline to control the rollout
• Maintains backward compatibility by keeping existing logic when feature flag is disabled

Benefits:

• Reduces complexity of the main RefreshService
• Enables independent scaling of pipeline refresh operations
• Improves observability and error handling for pipeline-specific operations
• Follows the established pattern from notification worker split

References

Part of the broader effort to split MergeRequests::RefreshService into smaller, focused services.

To Test:

• Have an MR with pipelines
• Turn on feature flag Feature.enable(:split_refresh_worker_pipeline)
• Push to the MR
• Check in the log/sidekiq.log file for PipelineWorker being executed
• Check that the pipeline for the MR is created
• Push with git push -o ci.skip origin your-branch to check that the params are passed through correctly and pipeline is skipped