Feature Proposal: Assign reviewer when pipeline succeeds

Summary

Add the ability to automatically assign a reviewer to a merge request only after the pipeline succeeds, similar to the existing "Merge when pipeline succeeds" (MWPS) feature.

Problem

Currently, when an MR author marks a merge request ready for review and assigns reviewers, reviewers get notified immediately - even while pipelines are still running. This often leads to a suboptimal experience:

1. Wasted reviewer time: Reviewers start looking at the MR only to find pipeline failures shortly after
2. Unnecessary back-and-forth: Simple formatting, linting, or test failures could have been caught by the author if they had waited for pipeline results
3. Notification fatigue: Reviewers receive multiple notifications - first for the review request, then potentially for pipeline fixes

Proposal

Introduce an "Assign reviewer when pipeline succeeds" option that:

• Allows MR authors to queue reviewer assignment until the pipeline passes
• Only notifies the reviewer once the MR is in a "green" state
• Provides a smoother review experience where reviewers can be confident the basic checks have passed

Suggested UX

• Add a checkbox or dropdown option when assigning reviewers: "Assign when pipeline succeeds"
• Show a visual indicator that reviewer assignment is pending pipeline success
• If the pipeline fails, the assignment remains queued (similar to MWPS behavior)

Benefits

• Improved reviewer experience: Reviewers only get pinged for MRs that have passed automated checks
• Reduced noise: Fewer unnecessary notifications and context switches
• Better author workflow: Encourages authors to ensure pipeline success before requesting review
• Consistency: Aligns with the existing MWPS pattern that users are already familiar with

Related Features

• Merge when pipeline succeeds (MWPS)
• Auto-merge
• Merge request approvals