Rollout: merge_immediately_when_no_pipeline feature flag

Summary

Rollout issue for the merge_immediately_when_no_pipeline feature flag introduced in !228142 (merged).

Related issue: #355455

What does the feature flag do?

When the merge API is called with auto_merge=true but no pipeline exists and no auto-merge strategy is available, this flag controls whether the API falls back to merging immediately (instead of returning HTTP 405).

Rollout plan

GitLab.com

• Enable on staging
• Enable on GitLab.com for a subset of projects/actors
• Monitor for errors/regressions
• Enable on GitLab.com globally

Self-managed

• Enable by default
• Remove feature flag