Add feature flag for async pipeline creation in NewMergeRequestWorker

What does this MR do and why?

This MR introduces a feature flag async_mr_pipeline_creation to switch pipeline creation to async mode in MergeRequests::AfterCreateService.

Problem

The NewMergeRequestWorker has a P90 latency of over 3 seconds, which exceeds our S3 criteria target of less than 1 second. One of the slow parts is the synchronous pipeline creation in MergeRequests::AfterCreateService#prepare_for_mergeability.

Solution

When the async_mr_pipeline_creation feature flag is enabled:

• Pipeline creation is performed asynchronously via MergeRequests::CreatePipelineWorker
• This reduces the time spent in NewMergeRequestWorker by offloading pipeline creation to a separate worker
• The existing execute_async method in MergeRequests::CreatePipelineService handles the async flow, including GraphQL subscription triggers for real-time UI updates

Feature Flag

• Name: async_mr_pipeline_creation
• Type: gitlab_com_derisk
• Default: disabled
• Scope: project-level

How to test

Setup

1. Enable the feature flag for a test project:

   Feature.enable(:async_mr_pipeline_creation, Project.find(<project_id>))

Test Steps

1. With feature flag disabled (default behavior):

   • Create a new merge request in the test project
   • Observe that the pipeline is created synchronously during MR creation
   • The MR should display the pipeline status immediately after creation

2. With feature flag enabled (async behavior):

   • Enable the feature flag as shown above
   • Create a new merge request in the test project
   • Observe that the MR is created quickly (pipeline creation is offloaded)
   • The pipeline should appear shortly after via MergeRequests::CreatePipelineWorker
   • Verify the pipeline status updates in the UI via GraphQL subscriptions

Expected Results

• Both scenarios should result in a pipeline being created for the merge request
• With the flag enabled, MR creation should be noticeably faster
• No functional difference in the end result - only timing of pipeline creation changes

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

Related issues

• Epic: &10991
• Related investigation: #417973 (closed)