Add auto merge process trigger on merge status change to can_be_merged

Summary

This MR adds a check for AutoMergeProcessWorker.perform_async when the merge status changes to can_be_merged and the merge request has auto merge enabled.

Changes

1. Feature Flag: Added auto_merge_on_merge_status_change feature flag (gitlab_com_derisk type)

2. MergeRequest Model: Added an after_transition callback in the merge_status state machine that:

   • Triggers when transitioning to can_be_merged state
   • Only fires if auto_merge_enabled? is true
   • Only fires if the feature flag is enabled for the project
   • Enqueues AutoMergeProcessWorker with the merge request ID

3. Specs: Added comprehensive specs covering:

   • Feature flag enabled/disabled scenarios
   • Auto merge enabled/disabled scenarios
   • Transitions from all valid source states

Why

Currently, AutoMergeProcessWorker is triggered when pipelines complete. However, there are scenarios where a merge request becomes mergeable due to merge status changes (e.g., conflict resolution) without a pipeline completion event. This change ensures auto merge is processed in those cases as well.

Feature Flag

The feature is behind the auto_merge_on_merge_status_change feature flag, allowing for safe rollout.

Test Instructions

Prerequisites

1. Enable the feature flag:

   Feature.enable(:auto_merge_on_merge_status_change)

Test Scenario 1: Auto merge triggers on merge status change

1. Create a merge request with a passing pipeline
2. Set up "Merge when pipeline succeeds" (MWPS) on the MR
3. Introduce a merge conflict (e.g., push a conflicting change to the target branch)
4. Verify the MR shows as having conflicts and cannot be merged
5. Resolve the conflict (rebase or manually resolve)
6. Expected: Once the merge status changes to can_be_merged, the AutoMergeProcessWorker should be enqueued and the MR should automatically merge

Test Scenario 2: Feature flag disabled

1. Disable the feature flag:

   Feature.disable(:auto_merge_on_merge_status_change)

2. Repeat the steps from Scenario 1
3. Expected: The AutoMergeProcessWorker should NOT be enqueued when merge status changes to can_be_merged

Test Scenario 3: Auto merge not enabled

1. Enable the feature flag
2. Create a merge request WITHOUT auto merge enabled
3. Trigger a merge status change to can_be_merged
4. Expected: The AutoMergeProcessWorker should NOT be enqueued

Verification via Rails console

You can verify the worker is being enqueued by checking Sidekiq logs or using:

# Watch for AutoMergeProcessWorker jobs
Sidekiq::Queue.new('default').select { |job| job.klass == 'AutoMergeProcessWorker' }