Auto-resolve stale todos when MR is closed

Goal

Complete the feature by adding close event handling. Reuses all infrastructure from previous phases.

⚠️ This completes the core feature - after this, feature is fully functional!

Related to #20637 (closed)

Depends on: Phase 1a, Phase 1b, Phase 1c

Implementation Details

File: app/services/merge_requests/close_service.rb

Add worker invocation after existing todo resolution (same pattern as PostMergeService).

Test Coverage Required

Location: spec/services/merge_requests/close_service_spec.rb

• Worker enqueued when flag enabled
• Worker NOT enqueued when flag disabled
• Todos resolved for all affected users
• Behavior consistent with merge path

Location: spec/requests/api/merge_requests_spec.rb

• API close endpoint resolves todos

Files to Change

• app/services/merge_requests/close_service.rb
• spec/services/merge_requests/close_service_spec.rb
• spec/requests/api/merge_requests_spec.rb

Acceptance Criteria

• Worker enqueued when MR closed
• Consistent behavior with merge path
• All tests pass
• No N+1 queries

Deployment Plan

1. Deploy to staging
2. Test close functionality
3. Deploy to production
4. Verify both merge AND close work correctly

Estimated Effort: 1-2 days