Mergeability check race with merged results pipeline creation

Summary

The mergeability check and merged results pipelines currently use the same ref and database logic. This means that they can conflict with each other. For example:

• An external mergeability status change can prevent a merged results pipeline from running
• A separate mergeability check can overwrite the merged results ref before a pipeline is done using it

See gitlab-org/quality/engineering-productivity/team#513 (comment 1971151198) for more details.

Steps to reproduce

Example Project

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

Allow merged results pipeline creation to proceed if the mergeability check fails after the merged results ref has been updated. !158250 (merged)