Draft: Add ff to ignore mergeability race when creating merged results pipeline

What does this MR do and why?

There is a race between merged results pipeline creation and target branch changes, as both can affect the mergeability status of an MR. When an external mergeability check "wins", the merge request pipeline creation currently falls back to a detached pipeline.

In this MR, we add a feature flag merged_results_pipeline_ignore_target_branch_race, changing this behavior to ignore a failed mergeability check if the merged results ref was successfully updated without conflicts.

The flag is disabled by default.

Part of #469380

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Before	After

How to set up and validate locally

1. In your GDK, add an artificial delay into MergeRequests::MergeabilityCheckService#update_merge_status as follows:

       def update_merge_status
         return false unless merge_request.recheck_merge_status?
   
         if merge_request.broken?
           merge_request.mark_as_unmergeable
           return false
         end
   
         merge_to_ref_success = merge_to_ref
   
   +     sleep 120
   
         update_diff_discussion_positions! if merge_to_ref_success
   
         if merge_to_ref_success && can_git_merge?
           merge_request.mark_as_mergeable
           reload_merge_head_diff
           true
         else
           merge_request.mark_as_unmergeable
           false
         end
       end

2. On a merge request, press the Run pipeline button

3. Push a change to the MR's target branch (e.g. master or main) before the artificial delay is up

Observe:

• When the feature flag merged_results_pipeline_ignore_target_branch_race is disabled, a detached pipeline is created
• When the feature flag is enabled, a merged_results pipeline is created